conebeckham wrote:.You do absolutely need an understanding of emptiness but according to my Sakya and Nyingma teachers the Mind Only school position is sufficient
That's interesting, Kirt...I assume that this is the Mind Only interpretation which does not posit the absolutely existing Mind? Or, in other wordds, Yogacara that posits mind as ultimately empty?
No it's valid for Mind Only that posits the ultimately existing mind as well. As Tamdrin points out in a later posting this will not result in full enlightenment but can establish one on the bhumis. So it can get people started on tantra and can even result in the bhumis but it is incomplete and insufficient for full enlightenment. In Sakya in particular it is basically treated as a provisional view (as are all views lower than Madhyamaka).
In comparison the lower schools view of Sautantrika and Vaihasika is not sufficient for one to begin tantric practice, even kriya yoga tantra. However I think Tsongkhapa suggests that kriya yoga practice can begin with a lower view (I think this is in HHDL's commentary in "Tantra in Tibet" but I can't look that up). In practice some view of emptiness is better than none (unless it's too far off and is nihilistic).