a dirty fuel filter will not affect your performance that greatly if not at all. a clogged cat an cause you to lose some power, expecially when you hit vtec and all you can feel is a sorta vacuum leak coz your exhuasts are backed up.
don't go straight into bolt-ons, you're right about checking your block first,
Engine swap, turbo, nitrous.
these should be the last things on your mind.
get a compression test like one of the members already said, or better yet, get a leak down test to see whats going on with your entire engine from head to block, vacuum etc. after that look into mantainance things like timing belt (timing, and might as well do the water pump), valve clrearances, piston rings, gaskets, sp's, cat, fuel filter and pump, cleaning your tb, air filter, and all that jazz...all these can greatly improve your performance and they're healthy for your eninge. you can probably do all these without breaking five bills. GL