Firstly the cover over the cams... It's called the cam cover.

Cam cover gaskets failing is common. Replace them with genuine GM parts (pattern parts don't appear to last in this application).
Ensure the breathers are cleaned out at the same time.
There's a guide to this job here:
http://www.omegaowners.com/forum/YaBB.pl?num=1152564281I don't see why a 3.2 exhaust wouldn't work but it's got 2 cats instead of one, of course. It may be more restrictive. 3.2 manifolds are reckoned to flow better than the 3.0 type so this is a worthwhile change. Some modification of the cat pipe is required to fit it to the new manifolds.
The main advantage with a stainless exhaust is longevity, of course. Some may flow better but, to be honest, the cats and manifolds are probably the weakest link so the rest of the exhaust probably has a limited effect.
I don't recall anyone having reported changing to hotter cams on this engine. There is also no information about those cams at all on their web site. If anything, they might give a little more top end power at the expense of some low-down flexibility but might require porting of the heads to give their full potential.
A better first tuning step is to chip the ECU, IMHO.
Kevin