I have a built ZF from RVO with a Dominator modified stock converter, and it has handled over 1000hp at the wheels through my awd territory. I'm currently using the same box at a bit over 500awkw and it shifts great. It all comes down to the tune with a ZF, as the wrong tune can quickly destroy a built box in no time.
Apparently the boost solenoid can get a bit sticky with age and some wd up it's guts will loosen it up. You can replace it with a Mac valve, although you may require a tune touch up, especially if you decide to use a 4 port version.