Stock around 350 hp on the high side. With a few slight alterations it can take alot. Just redrill some of the throttle plate holes, add an overdrive gear from an F150, and add some more clutch plates.
With this done it can take over 400 hp. This is according to the man who designed the tranny. Jerry W. He wrote several articles over at www.tccoa.com. And he is a reguular at modulardepot.com.
If you are doing a power adder the tranny slipping might not be as big a problem as it hitting the rev limiter. The tranny takes about 500 rpms to make a shift, so the cpu tells it to shift at say 5000 rpms and the speed of the car stock will have it shift at 5500. But with alot of gear or a power adder, the cpu will tell the tranny to shift at the same time, but the car will accelerate so fast that it will hit the rev limiter before he can actually shift.
A shift kit or even better the J mod (found at tccoa.com) will alleviate this problem and decreace time between shifts.
Also, even if you aren't doing heavy mods get a transmission cooler. The best one is the B&M (check my sig). This will basically double the life of the fluid and in doing so the life of the tranny itself.
Good luck. If you have any more questions feel free to ask, I don't know much more than I posted here.
P.S.- my TranGo'ed tranny has taken 3 bottles of a 75 NX shot so far with no problem at all.
I've heard that it will handle about 400hp fairly well. Anymore than that and it could be a couple of days or a few months, but it will go.
My tranny seems to be holding up well enough, but i haven't broken the 400hp mark, i'm probably somewhere in the 360-380 range,and i haven't put too many miles on it since the turbo.