I know of guys who have gone the 3.2 ---> 3.4 route and say the bang for the buck was pretty low. One guy who comes to mind is member 89911 here on the forums. Both he and his brother track their cars and I recall him saying that his built 3.4 twin plug doesn't feel all that much stronger than his brother's 3.2 Euro 10.3:1 single plug build, and if he had it to do all over again he would go the 3.2 Euro route. So, since you've already got the higher compression Euro 3.2 pistons, I don't think you'll see a big gain. Yes, there's "no substitute for displacement" but the gain is going to be somewhat small.
The stud thing is weird. Porsche has been all over the map with the stud material on these engines. 930 had all dilavar, 911 had a combo of steel and dilavar, 964 comes along and uses all dilavar, then the 993 uses all steel, then the 993 twin turbo comes along and uses fully threaded all dilavar. I did some crude research on this a few years ago
Broken headstuds. Inevitable?
The dilavar discussion can get pretty crazy sometimes
993 Head studs, two types?