i've got a k&n pannel with uprated cold air feed from the lower grill. think it sharpened the throttle response a touch and does allow the engine to breath a little easier but realistically you get 1-2bhp gain max. i do like the fact i can clean it though when it gets dirty!

definately sort the exhaust leak, you'll be amazed a how they can screw your performance (depending on the severity of the leak and its location).

other performance increasers are a cat back exhaust (preferably swap to a better manifold too removing the cat, but then have the hassle of refitting the cat for mot) cams and you can ge the standard ecu remapped to optimise those mods. Neil @ fast ford has just done these mods and got around the 160bhp mark wich is roughly st170 performance. throttle bodies would take that further to about 190bhp but thats really the limit n/a unless you build a screemer of an engine that will rev to 9k.

personally though i'm going to go turbo route with aftermarket management. the standard engine (bar fitting some uprated valve springs) can happily be boosted to 240bhp, certainly thats what the powerworks supercharger kits took them to.