Yeah for £1500 you're never going to get anywhere near 300bhp. Mine had 250bhp with a mass of expensive parts. Dotty on here spent a small fortune on his and was nowhere near that.
Turbo or Supercharger setups will set you back about £5000ish.
To get the car remapped you'll need either a K100 or KPro. The first will be around £750 including a map, the latter will be over £1000 new.
You're better of concentrating on handling mods for the car, it will make it a far better drive and more engaging.
Go for springs from Eibach or Spoon, along with camber arms and bolts and get a good Fast Road Set Up done. Speak to TGM, TDI North, ABP etc. They are all specialists in Red H's so will know how best to guide you.