drill holes in the airbox, then its all safe and just sounds better

keeping all excess weight out of the car will of course make it faster in all aspects
(space saver wheel instead of a full alloy for example)

and maybe a little bit of heat defence as ive heard focus' suffer from heat soak!?

maybe you could make up a pipe for the exhaust that will get rid of one of the silencers, then it wont be too load and wont be too expensive!?

another mod i thought of, not quite sure if this would work or not but, the drivers side mat and carpet, either taking them out or trimming them a little bit so the throttle can be pressed further, again not sure on that but ive been wondering if it would work for a while