As you likely know, I have retrofitted my US-spec 164 LS (Super) with a 156 GTA 6-speed and it has been taking some punishment very well. I don't abuse it, but I
am running 300-plus horses through it from a Glenwood Motors 3.45 litre 24 valve with over 300 horses at the crank and LOTS of torque (sampled often)!
I HAVE heard Dawie say however, that the 5-speed 164 box is stronger that the GTA box (and as such - he ran the 156 GTA race car back home with a 3.7 litre build on the 164 box for the last few years of ownership)! In fact, it may still have that box in it today...!