3.8L

Wow that woke up the V6 forum.. don't forget you can get the 3.8 pumped up naturally from places like Super Six Motorsports; they claim you can pick up from 75-100+ horses with cams, ported heads, etc. Although, that won't be cheap, and the original question was for cheap horsepower.
I'd say start with dual exhaust, cold air intake, T-lok and 3.73 gears, and see how that feels. You won't be eating GT's, but you should feel a good difference.

Someone correct me if im wrong but you should have both heads shaved at the same time... not one shaved and not the other.

It will bump the compression ratio a bit I beleive, dont think you want one side higher than the other.

Also, make sure you change the head gaskets now as cheap insurance.

yeah, if you are shaving one head, you'll definately want to shave the other an equal amount. Never take the head off without replaceing the headgaskets. They physically crush to seal and likely won't seal again.

Things to consider for the supercharger: Don't shave the heads anymore than is needed to make the head flat. You don't want more compression if you are going to boost it. Also, you'll have to tune it in some fashion. This means adding fuel and taking out timing so it is a safe tune. Keep in mind the factory supercoupe probably got forged pistons (I'm no supercoupe expert, but I am pretty sure they did or should have). Of course, forged is much stronger and will tolerate a few pings if your tune isn't dead on or you get a bad tank of gas.

I've seen some impressive results from running a single Holset turbo on the 3.8 (such as the HY35 from the dodge desiels). Something to consider.

I've got a 2.3 turbo. I origionally put a turbo on the NA engine... it had cast pistons and rings. It held together fine and ran strong, but one day getting on the onramp the vacume reference popped off the FRP (went lean). I didn't hear a single ping, but I broke both the top and 2nd compression rings in one cylinder.

Just as your case, it might be easier to just swap in the entire supercoupe engine with computer too. It is beefier internally and already has bigger injectors. It will save you money and a rebuild in the future.

That's all I've got for ya. I really do recommend taking the whole supercoupe engine instead of just the supercharger.

There is definately something fun about making more power with less cubes/expectations. If I go out and run a 13.X in my 2.3, not a single person will say it should be faster or I'm doing something wrong. I'll honestly be disappointed if I can't get a 12 at better than 106mph this spring though.