I replaced my OEM filters with K&N's, I think I could tell a slight difference. It seemed like you could hear a little difference too. In my opnion the main restriction in the OEM set up is where the intake tube's go into the grill area. There is also possiable restriction in the "Y" pipe when you have an intake hose off because of the shape of the opening. But then again when you really think about it, the opening is oval, but it is the same size as the round part coming from the filter housing. One other thing to consider, when you are blowing through a tube, pipe or what ever the fewer bends or any other obstacles the easier it is to blow threw. But when you have a pullied up supercharger sucking like hell, I think the only thing you would really need to be concerned about is actual restrictions. BUT THAT'S JUST MY THOUGHTS. I could be full of poo.
[ EDIT] I look at it like this, for a supercharger, it's the volume of air avaliable for it to suck.