The filter and throttle body will aid air intake and the decat will help with the exhaust gases. People have been known to gain from having a chip with only these mods to help sort the fuelling out. But in most cases the standard ECU will sort it out adequetly.

Its up to you if you want to spend the money but dont expect too many gains from those changes. The best way forward would be to get hold of an ST200 upper and lower inlet manifold to really sort the intake out.

Jeff