Dude have you seen a stock OBS DP? You're thinking of the SD exhaust....I wouldnt even bother changing it;. The stock works good enough/ its been dyno proven.
To the CSWASS yes you can put a larger than 3" DP on and some guys have done it. The challenge is getting it passed the firewall/floor pinch weld. Some guys do a body lift and a significant amount of cut weld of fore noted. Greater then 3" is really not needed unless you are running huge ponies. 3" DP and stock exhaust including stock muffler will support 400-450 HP....I say save your dough and get a 3" DP into 4" and clean out your cat and use as a resonator into stock exhaust for now. Then when you need to replace your stock exhaust go to 4" system.