I really don't think your going to get 300hp CHEAPLY outta any of those cars. And I wouldnt buy a 350Z unless it doesnt snow around your area, A buddy of myin owns one, and had to buy a beater for the winter the new Si's kick ass!
out of all of those i would buy the 350 if you can afford it. but if not then DEFINATELY get the RSX over the SI. MUCH better car all around. not to mention nicer the easiest way to get about 250WHP from a type-s is through a jackson racing supercharger. but hey cost about 3 grand... you could turbo for about the same price and probably make even more than that. but really the question is... how much do you want to spend? for the right price you can make 500WHP and SPANK Z06's but i can say that for about 1500 you can def bump the power up to around 240. thats intake/header/hondata reflash. but thats not 240 WHP thats engine. and dont spend the money on K-Pro if your not actually doing anything to it that isnt a serious power increase. the stock computer can compensate for up to something like 230ish HP. hope my book helps