These cars benifit more from handling modifications rather than engine power modifications. The better you get the handling the more speed you can carry through the corners and onto the straights.
If I were you the first thing I would consider is to renew all the bushes, track rod ends, steering rack bushes etc to tighten up the car. It will basically feel like it's factory fresh, which gives you a good starting point to build on.
Stickier tyres, decent coilovers, rear anti roll bar would be next.
Strip the car of any excess weight. Lighter = faster. Plus it's free.
If you must do engine mods then I'd consider an RS Tuner map, a performance air filter or a Ph1/Espace V6 airbox if you can find one and an aftermarket exhaust to help the engine rev more freely. An exhaust will save some weight too, bonus.
If you wanna get more serious, and spend more money, then solid top mounts and solid engine and gearbox mounts.
All depends on your budget really, but the costs of cams and ITB's (if you want power) would be more beneficial spent on the above IMO. Although it may not be increasing your car's power output, it will still make your car faster round the track. It's not all about the BHP.