If your after cheap gains, a panel filter and removing the pre-cat would set you back less than £100 all in all (less if you do the pre-cat yourself).
Or go for all out performance as Wooosh suggested above.
Depending on condition, fuel quality, tuner etc. . you should make around 245-250bhp on a car with the mods ive suggested above. I got 248bhp and 311lb/ft of torque from R-Tech on my old GTI, although since then Niki has improved his mapping techniques so the tiv mapping may see you a couple more HP