Your a lucky man, these cars make absolute beasts once remapped.
The first thing I'd do is change the DV valve for the new, uprated version as your old one won't cope with the extra power, next I'd fit an ITG panel filter (or ITG Induction kit if thats how you like it) and a remap, this should bring you close to your 300 bhp - if you like them you could also add a cat back exhaust system for a little added drama in the noise area
BTW ITG is my own preference but there are many good quality air filters on the market, the only exhaust systems I really rate are Akrapovic but your looking around the £1200 and they don't do them for the MK5 yet...
