Is there any noticeable performance gain with the air filter change? I've been told by a friend to buy a HD kit that replaces the backing plate and filter for increased air flow. But I haven't seen anything that says if it uses the original cover or not.
You can buy the necessary HD parts without buying the cheap HD paper filter, which will fall apart eventually. There's a thread on here somewhere that is "Roll Your Own" filter. You will need the backing plate, some O-rings, etc.
Or, there are several aftermarket pieces that will work...Arlen Ness Big Sucker; Doherty Power Pacc; Zippers; etc. Most of them come with the K&N filter and all necessary hardware.
If you are running the stock filter with the enclosed backplate, putting the SE backing plate and K&N will definitely help the motor breathe better. That and an exhaust will give you a good 10% increase in both HP and TQ. That is standard procedure on all the Twin Cams that came with the closed box air filter.
On the '05, fitment of the stock cover is not an issue. The K&N filter will last you basically for the life of the bike with proper maintenance.