There are way to many positive threads about this transmission so i'll add one:
My transmission completely went at 50,000 miles (while i still owed 10,000 on the car). But the guy at the tranny shop says that either it was an old transmission that was swapped in, guy before rolled back the clock or he did a 12 minute brake torque letting the tranny shift back and forth between 1st and second. The new tranny and torque convertor are great though, feel extremely strong, I definately think the tranny could hold up with some added boost.
Yeah I just got a reman 4r70w transmission put into mine and she's working like a charm now. (for anyone wanting the place I got mine its just https://reman-transmission.com/transmission-codes/?code=4R70W (it took me a TON of digging to find these guys but they are great))
All I had to do was take it to the shop to get the 4r70w put in.. (too bad im not that much of a car guy or i'd put it in myself )
Most HP that I ran with the BONE STOCK tranny/converter was 524rwhp and it held up great. The key is heat. Keep the tranny as cool as possible will extend its life. I ran a HUGE tranny cooler.