as said above,
if you buy a normal GTI(K03 TFSI engine) with 197Hp/207Ibft,
you would require to a performance downpipe, performance intake, performance intercooler, uprated fuel pump and a good stage 2+ remap to make yourself reach 280Hp/330Ibft. to hit 300Hp you would require water methanol injection or nitrous really, or a go on superchips dyno for some free horsepower
the above would cost around £2000. but searching second hand you can massively reduce thatoutlay.
if you were to buy a golf GTI ED30, leon cupra or S3, a remap alone (£500) will see you at 280-300Hp. add an exhaust, intake and stage 2 remap and you'll hit 330Hp, add a fuel pump and you'll be pushing upto 360Hp/360Ibft for the same price as the std GTI.
in short,
£2000 of mods on a k03 TFSI will gain you an additional 70-80Hp and 100-130ibft.
£2000 of mods on a k04 TFSI will gain you an additional 100-130Hpand 100-130ibft.
the k04 engine is pretty much untouchable for tuning poential in this price range. even the focus RS when fully tuned with identical mods wil struggle to keep up with a k04 VAG, and thats considering it started at 300Hp as well