I bought my na for the simple reason that i could not afford the tt when i was looking for a supra, its true ive probably spent more now than what i could of bought a tt for BUT i havent had to spend a big wad of cash in one go, it`s been a gradual process and over the few years of ownership ive managed to get tt power out of my na + plenty more potential.
Ive posted this very question myself in the early stages of ownership (not on this site but others) and i have had the very responses you talk of, some are all in good humour, some more serious but dont let it put you off, as you say a decent exhaust system with both cats removed will be a help and the heat shield a must if your going to run aftermarket filters, also try finding someone who can fabricate you a custom intake pipe and remove those 2 horrible silencer boxes, i noticed a difference when i done this to mine.
removing the aircon and viscous fan will take a bit of load of the engine but dont expect huge noticeable power gains from this.
I think 300hp is a little out of reach without mega bucks like some have said but getting a bit extra out your n/a shouldnt be to much of a problem.