Welcome to the new and improved Proflex / K2 Tech Forum!
I have a 130mm stem because this is a Medium frame and I'm 6'1"... and I prefer flats to risers for purely aestitical reasons - so much cooler! ;o))) Also, I heard of problems of Barend clamps cracking carbon bars...Discs are a difficult choice... I bought the Rims and Magura Hydrolic rim brakes purely for this frame... The reviews say they're awesome...
No way, no how. You'd be crazy to put aero bars on your mountain bike. I've seen it done, but only by a guy who rides on the road a lot (which is pointless and not very fun on a mountain bike anyway). Off road, even on a dirt road it would be dangerous. Plus, the reach to your bars is actually longer on the mountain bike. At least on mine, my reach to my bars is about the same as the reach to the hoods on my road bike. Putting aerobars that far forward would make them kind of far away for your elbows to rest comfortably. Plus, as a time trialer, I'm sure you realize that you lose a ton of your steering precision. Even a small rock on a flat section could cause you to lose control. If your aerobars are like mine - with spring loaded pads - they would rattle too much anyway..