Exhaust and filter will make BUGGER all difference to a standard car's performance/but at least you'll help 'someone' with this months mortgage payments.... Chip off ebay or one of the classifieds (about £40) 31 actuator
(about the same) and a trip to Jamie at Jamsport (£125) should see you
knocking on the door of 185bhp.
Then, 2wd Cosie discs (£50/60 anywhere) and brackets to resite your standard calipers (£40 from Clint Gaskin at Turbosport) goodridge hoses (£35 anywhere) and junk the standard 'abs' now you'll be able to stop (fairly important) as well as go.
Once your go above this it starts getting expensive/clutches+gearboxes etc
Good Luck