If you are not fussed on performance, why spend the extra cash on Uprev, HFCs and plenum? just buy a good quality exhaust, and fit a air filter yourself. Obviously the Uprev will give you more performance wise, but think its more the drivability that you'll notice.
Milltek is very quiet...hence why I'm going to try decats. If the sound is fine (not too loud..LOL), may look at HFCs. But as you mentioned, they aren't cheap so want to be certain first - hence the decats.