Well a few of my mates have completly destroyed the box from an RB20DET, mine is still holding together, but i am not making mountains of power either. there is a reason the 20DET box is cheaper than the 25DET box. I will be keeping my RB20DET box purely for fitment reasons, when this one dies, i will be getting a replacement cyro'd, as this has proven in a few cars i know of to handle abuse quite well.
most times i see and RB20DET box destroyed is when someone has put an RB25DET in their R32 and kept the 32s box, many people destroy the boxs.
on an RB20DET the box would be fine, and its not about how much power the box will handle, its about how much torque it will handle.
the RB30e you mentioned in VLs and such are not exactly powerfull, and i have not known an RB20DET box to last long behind a turbo version. most VL turbos use a jericho (spelling?) auto.