Ok seriously, according to alot of information out there from the big brands the apexi ones flow the best and the K&N ones filter the best. Weather you'll actually notice a difference between the two I doubt it. People who say they can notice a 5kw difference in power are full of shit. I've seen many people on dynos going from the orginal box, to a modified box to a pod to nothing on consecutive runs and majority of the time on stockish cars the original box makes the most power....go figure??? As far as I am concerned, put a pretty on on there so it looks nice and be done with it
as to you last question, going to an open pod from a standard box you'll probably loose a bit of power cos pods suck in more hot air than the box. its obviously gonna effect you more in city driving where your car heats up as opposed to a track......