My dad has a 02 Bonneville SSEI, yea i know its not a GP but its pretty darn close. He wants a little more kick out of it, and asked me what to do, i said either get some headers, a smaller pulley, or a ECU change or tune.
He does NOT want to go into the engine, he wants bold on mods for a little more power, nothing rediculus, just somethign to throw him back in the seat a tad more.
the 3800 is an excelent engine with a lot of potential. Theres a website, 3800performance.com maybe, google 3800 performance. They sell the supercharged engine upgrades in packages and stuff and its all sorted by car so its easy to use. Check it out, you should find what you need there and approximate hp and performance gains.