I don't think it matters what you got as long as you like it and you always have the ability to make it unique. I'd take an LS1 anyday, but living in the midwest and being a college student, it just isn't feasible right now. The aftermarket availability of parts for the LS1 is nuts.
That being said, 300whp on average will not take your usual grand prix into consistent mid 12's. There are a few more things to consider, how much weight reduction and such and the way you are asking sounds like its in the hands of an average kid, kinda bold to say it will run mid 12's 300hp. Given that these engines are not dyno queens in no way, they do make some good e.t.'s for the power they have.
There are a few that I know have gone high 11's with minimal mods and a lot of weight reduction +tuning. Ron Vogel went 11.8-9's I believe with 1.9 rockers and intercooled setup, alot of tuning, STOCK tranny. That was probably less than $2k in mods but they made a lot of things themselves.
Personally, I know of a S1X cammed gtp that has run a 12.3 (Gooch). He is probably in the 300whp+ range. His mods are s1x cam, genV s/c, N* tb, 42.5# inj. He probably had less than $2k into the mods including hptuners, but then the tranny went out.
Those are a few examples but I would have to say consistant mid 12's with 300hp is not going to happen for everyone.
