When I had mine it had a k&n induction kit and I made a custom exhaust back pipe (used a tdci back pipe that has no box in and welded a tip on) and got some 30mm springs. I personally feel they are underpowered which is a shame as they are a nice car. I'm not convinced my vct was working right though. My advice is do handling mods as it was the best handling car I've ever had and it has got a decent brake setup as standard too so you can carry the speed well.
From what I read when I had mine the vernier pulley is the key to releasing power. The manifold is a good design already and they have a sports cat as standard so I'd spend the money elsewhere rather than on a manifold and decat.