although i dont like using the stuff, nitrous would have you at 400 hp in a few hours.. you would be running high 12s with the car as is.. but nitrous has its bad points as well as its good points.b...
forced induction would also have you in the 12s, but the work would need to be custom,,as there are not many FI GSs..
i have previously twin turboed an older-93- sc400, and can vouch for the increased fun factor, and the suprising ability of the lexus v8..
but, depending on the shop, that can become an expensive endevor.. so let the buyer beware..
as long as you know what you want, there are alot of shops that can help you out.. but, finding a shop you can trust not to cheat/overcharge you, or ruin your car --can be tough..