A 600 hp is pushing out about twice the air as your 340 and has only a 13-20 hp difference between 2.5 vs 3. They said 3” was about par with open headers.
So how much you lose by going 2” ? Instead of 2.5” probably about the same. Unless you put some power killer mufflers on some are terrible and might kill twice that. Testing is the only way to know for sure, I wouldn’t go with 2” but it don’t seem like you overly care about power.