I guess you haven't made significant rwhp w/ an L28 yet...otherwise, it would make perfect sense to go to a beefier tranny. Changing trannies every few months gets old fast, not to mention worrying whether or not it will blow up on you at the track.
I have made nearly 400 hp through the stock L28 tranny too (calculated from my 1/4 mile mph)... and there are guys making 400+ hp w/ stock KA trannies, but ask those guys how often they have to change them out. Also, torque is a bigger factor than horsepower and the L28 will make more torque than an SR at the same rwhp levels. The idea is to get a stout transmission that you can rely on. If you are worried about robbing the car of horsepower, you can turn up the boost another 1-2 psi, not having to worry about the tranny so much, and ... problem solved
BTW, I dropped off the L28 tranny bell housing to Mark today. He'll get started on it when he has the chance. I plan on bringing the adaptor w/ me to SEZ, if the L28 adaptor is not finished, then Mark said he would lend the SR adaptor attached to the Z32 tranny to me so anyone coming can get an idea of what it will look like. The wheels of progress have begun turning!!
mike