I am sorry ,but this has annoyed me a bit.
I have been riding trials for nearly 14 years, I have tried and experimented with all kinds of things on bikes (made my own rigid forks from suspension parts, welded rims together to make light wide rims, cut and shut disc systems etc).
It annoys me greatly when someone with not much trials knowledge waltzes in and thinks they know best....you don't!
I am a profesional wheelbuilder for a well know bike company, you can't say spokes don't flex. the rigid mtbers may be making up excuses for not making it up the hills, but that isn't trials. Trials demands a whole new level from nearly every part of the bike. The flex in spokes is enough to take the "feel" out of the rear wheel when you need to be pin point accurite 10ft up on things barely bigger than your tyres. Unless you are riding in these conditions, don't make asumptions.
As for actual braking power, I had a Saint, and very powerful it was too, I have also had a BB7 and that was on par with the Saint plus it was more adjustable. We use stuff that works, get off your high horse, just because we use cable brakes doesn't make us un educated morons. Just because I can't bleed a car brake doesn't mean I can't bleed a MTB brake.
Can I hazzard a guess that you are an enginner by trade?
Discs on mod are fine, you can use smaller rotors as the wheel doesn't have as much mechanical advantage.