I am not as familiar with the 3800 Series I'*, but I have gotten to know a bit about the 3800 Series II'*, and the main thing I suggest is getting the Lower Intake Manifold Gasket replaced from the known to fail nylon 6,6 lined gaskets, and go with either the Fel-PRO, or AC Delco 2nd design aluminum ones.
Also, make sure you flush/change/bleed your coolant regularly, as well as service all other fluids regularly.
If your water pumps are original, and you have over 100k miles, then if it is in your budget, I'd consider replacing them with the more expensive AC Delco water pumps with stamped steel impellers, and aluminum framed water pump gasket.
As far as boosting performance, are you looking for mod advice, like machining superchargers, upgrading cams, valves, etc?
Or are you just looking to get the best performance out if them the way they are now?
2006 Pontiac Grand Prix GT with SC
1996 Buick Park Ave NA