Is the g50/20 a standard carrera box.I think it is.
I ran a similar box for a while in my 1st ultima and it was fine with 350 h.p but a 50/50 became available and when I upgraded the engine , I did the box at the same time .
400 H.P will have a lot of Torque and this is the deciding factor and also how you drive it.
Whether you have slicks, whether you drag it away, age and miles on the box, there are lots of factors.
It's no big deal to replace it if it destroys itself but I bet it wont knowing porsche
keep us posted