Short answer: Yes and no. Depends who is doing it, when, and why.
*I am sure you noticed you posed a question in the beginning of your post...then answered it yourself in the end of your post. So it isn't clear to me what you'd like to know. Was this deliberate?
In any case, my long opinion-answer...
I am biased on the topic. I find it inspiring and motivating to practice when I think about these monks. These are not athletes doing athleticism. They are ordinary people accomplishing something very difficult and extraordinary...and causes me to self-reflect on what I am doing with my own life. How much am I committed to a spiritual vision and path? How much will I persevere despite difficulties? How much gratitude do I have for the community- both sentient and natural world - that supports my life? How can I give back and respect it?
The Buddha did not reject the practices you cite as extreme- sleep deprivation, minimized food intake and enduring extreme conditions. He did approve of them specifically as 13 non-mandatory ascetic practices for monastics to take up called 'dhutangas'. Each is an antidote to certain attitudes and attachments. Asceticism wasn't rejected wholly, just the self-mortifying kind. Other examples from traditions- the Chan monastery of City of Ten Thousand Buddhas has a requirement of their monastics to not lay down and to only take one meal a day at noon. Some Thai monasteries and others have similar practices as well. Often enough Seon monastics in Korea like to offer their fingers in candles to Buddha. I think 'ascetic' parallels can be found in most streams of Buddhism. There are also stories of Chan monastics in China (oh and 4 in the US!) going on long extensive walking or bowing pilgrimages. It might also be noted that before viharas were established monastics in Buddha's time were walking all day from place to place. They were wanderers. I wonder what the average kilometers a day was as they wandered from village to village? Extensive walking practices aren't new or special to Tendai. People have also been doing marathon sitting...but that isn't nearly as exciting to document as walking in circles on a mountain.
I don't know if in your tradition you have the traditional Vinaya studies of Buddhism to compare with. Some folks are not aware of these dhutanga teachings. All forms of Buddhism today are divergent in their own unique ways from any constructed "original Buddhism" of the Buddha's time. Time, place, culture, practitioners etc. have all had their influences, so some differences occur.http://en.wikipedia.org/wiki/Dhutanga
for reference in case readers are unaware of this aspect.
The primary example of self-mortification from the hagiography of Buddha was that he was eating one grain a day for quite some time and nearing death. He was completely emaciated. This was in a culture where some may have been trying to out do each other on physical feats like holding arms up until they wither or spending day surrounded by fires or cutting off their genitals or what have you. The intention, and outcome, of those practices and contemporary ascetic practices are different- both in traditions, institutional views and individuals.
Asceticism is very relative I believe...in my own part of America in this century...many people consider monastic practice- "middle way practice" - to be ascetic, archaic or extreme. Some people I know feel celibacy, not drinking and being vegetarian is really too much. Not having a monastery with central heating is extremist. Asceticism 'then' and asceticism 'now' are different. I feel both in practice and in social context/meaning. Kaiohogyo doesn't seem extreme or even ascetic to me. It does seem like it would be internally confronting and I am interested in how it ( like many other Buddhist practices) becomes a sandpaper of self-reflection than softens our rough edges and polishes us up.
A whole discussion about what constitutes asceticism, when and why it is useful or harmful could be made. It can cut both ways and can be as helpful as it can be dangerous for sure. Ascetic practice isn't for everyone...but not all dharma doors are. Which I believe is why the Buddha made the 13 dhutangas optional rather than mandatory.my thoughts on kaihogyo specifically:
The statistics you cited are not quite accurate. I imagine they were drawn from the "marathon monks" book? According to what I know from my teacher...that books gives a slightly sensationalized version...it isn't nearly as "harsh" as people make it out to be. Roughly...they mostly walk 30k or 40k a day for about 800 of the 1000 days. Not 80k until the very end and only for 100 days. They do not walk 17 hours a day. More like 7-8 hours or so...sometimes longer. It isn't marathon running. They aren't 'marathon monks'. They eat more than a rice ball and a bowl of noodles in a day. They also nap quite a bit. The last people to have died was in the 19th century or earlier. That is more an expression of determination. Not just anyone is allowed to do 1000 days. It is not considered a practice for the masses by any means. More commonly people do 100 day practice and may apply for the longer practice. There is a certain level of screening that takes place for suitable candidates, it isn't beneficial for everyone and not promoted to be standard practice. The fasting period is brief and intense rather than a long drawn out slow death practice like the Buddha was involved in. It is done quickly and not a sustained "lifestyle". The goal isn't death but has been described as 'near-death-like'. I think after some generations of folks practicing this they've got it down to a controlled technique. If you've heard of Tibetan Nyung Nays...some people practice it for 200ish days in a row at times. Nyung Nays can easily feel death-like at points during the practice. Doiri, as it appears, is a bit more intense and much shorter but the practice has some similarities to Nyung Nay (sleep deprivation, no food, no water, lots of mantra).
Either way the aim isn't self mortification, rejection of the body, physical endurance or being a 'spiritual athlete'. People who focus entirely on that have missed the internal process of devotion completely. Kaihogyo is equally for the people that support the practitioners and once a monk has completed the practice they are to share their practice and serve a community. They become encouraging for others to continue to persevere and accomplish the practices of a Bodhisattva with great energy. So a major part of it is perseverance
, not enduring. Bringing circumstances to the limit so the practitioner must rely on the power of Fudo Myoo, their own will, faith and spiritual strength.
Of course some Japanese cultural aspects find their way in and things like "kill yourself if you fail to complete the commitment you made and let everyone down". Luckily they've dropped that practice and it is more a formality. Much in the same way in Vajrayana you swear not to reveal the secrets or your head will split open. Or when the Buddha told Brahmins in one of the Digha Nikaya Suttas they'd better answer him truthfully or Vajrapani would split their skulls seven ways. Perhaps it gives an idea of the seriousness of the situation and some people just got zealous. Like burning your fingers off because the Lotus Sutra says so. Or cutting your eye lids off so you can stay 'awake'. Maybe some people are Bodhisattvas though and see the benefit in doing it for others.
Suicide is very unBuddhist. It is one of the things among many a Bodhisattva may do to help manifest the Dharma and transform people's hearts. http://en.wikipedia.org/wiki/Thich_Quang_Duc