ST220 or ST TDci, as you mention an intercooler? Of course, the petrol is non turbo so no intercooler.

There are only a few things you can do, and they make a nice difference together but wont give you an extra 30-40bhp!

My choice would be a Miltek or JP exhaust. IF you want some induction roar, get a K&N 57i, retain the lower half of the airbox and modify the front feed so that it forces air down the inlet for your cold feed. You can also fabricate a shield to reduce the heat soak a bit.

Otherwise keep the standard airbox, K&N element which will last forever, and depending on age of car, if post 2003 make a feed for the spare outlet on the airbox, this makes a massive difference!

Finish off with a Bluefin, which will add a bit more fuel in places and sharpen up response, however it will not give you a huge amount of bhp, a Dreamscience Strategem X3 is the best for these cars but are only available 2nd hand.