I would say that adopting an uncomfortable posture to induce pain would be the wrong path of self-mortification. The three characteristics of anicca, dukkha, anatta can also be observed in pleasant sensations, and in mental states, so there is not special merit in observing painful sensations.
I think that the best way to use limited time more effectively is to improve your preparation before sitting. Choose the right time and place, keep the place as clean and quiet as far as possible, with good ventilation, etc.
was one of the vipassanā teachers described in "Living Buddhist Masters" by Jack Kornfield. The Sayādaw is no longer living as that book was first published many years ago.
• • • • (Upasampadā: 24th June, 1979)