Your best option is probably a K & N panel filter. As long as you get the correct one for a MK7 it just slots into the existing air filter box.
I've used the same one on my three Fiestas now, just swapping it out for the original air filter before selling, and only had to clean it once using the K & N Recharger cleaning kit.
I couldn't really say it sounds any better as I've never really noticed, it also won't make much if any power difference. However, it is the one that Mountune fit on all their upgrades, so if it's good enough for them it must be good
Have a look on ebay for the best prices.