you have covered the basics. next step engine mod wise starts getting EXPENSIVE!, with injectors, bigger turbo, fuel pump, proper ecu and tune, coil packs, etc the list is endless.
as 4show said, i'd now focus on suspension, brakes, wheels and tyres, and do them properly with quality parts!!!
300hp is fine to want, but if your car cant use that power, than there is no point in having it.