I can not believe nobody has mentioned.
If you change the air filter to one that gives you more air flow (and most will) you need to either have the dealer remap the computer or get a fuel tuner. such as pcV or fuel pak. you are giving the bike more air therefore you need to give it more fuel to run right.

 

I'm also considering the joker intake....what specific rain sock fits it? Also curious about that comment hurting the performance?

 

I remember reading that comment about the guy losing power and I find that a little hard to believe since the ecu on the newer sportsters is flexible and will adjust a small amount when the air/fuel ration changes. I wish I had my dyno sheets available to me right now to prove that even without a remap or a fuel pak it will gain power after a bit of riding. I have the JM intake and have been nothing but, happy with it. At first the bike was rough it sounded like a large single cylinder engine then, after a few minutes of running it adjusted and sounded great. The current exhaust I have on there is quite loud and even with that I can hear the intake sucking in air. It sounds like a small turbo spooling up. As for size my knee still hits it a little bit but, no where near as much as it did with the stock intake.