I agree, the sleeper factor of a Lumina is something to behold. Not many people expect a Grannymobile to do anything.
60degreeV6.com has a number of performance resources for your engine. Though I'd start with the 3.4DOHC (if you are VERY mechanically inclined, see below) and a beefed up 4T60E or even rip a five speed from some other GM FWD. My old shop instructor modified a 1998 Monte Carlo with that setup, stole the trans from a Saturn and a 3.4DOHC. Wasn't half bad.
I say mechanically inclined because the 3.4 is notorious for chewing up knuckles. From the alternator to the cams, it's just a bitch to work on all around. Great for mechanics, we make tons off the people who bring them in for service.
True, there isn't just a whole lot you can do to a 3100, though I've seen people pull the 3800 series II and drop it in with the supercharger sticking up. Let your imagination take over, as long as your pockets are deep enough.