if its just breathing in warm air, then it will have a negative effect on performance yes. you want to have your filter placed in one of the coolest places possible in the engine bay, ideally as far away from the cylinder head as possible. ideally you want to run a cold airfeed to it as well.
my 57i is mounted straight onto the plenum chamber above the throttle body, with the airbox taking air from in front of the radiator and blowing it straight onto it with the original pipework, no negative effect at all.
getting as much cold air through the filter is good in theory, but remember that the air going into the combustion chambers is always going to be roughly 60-70'C because the head and the intake valves, as well as the barrel walls and piston crown, are always going to heat it up as it is being sucked in. warm air is going to be better for fuel vapourisation than the coldest air possible, or the fuel wont vapourise properly and stay in little droplets, which doesnt burn as well and chokes the exhaust up after a while.
i had my car for bout a month or 2 and only had just all standard air box etc, but sounded well, but after we took it apart we realised it was drilled, so i dont know what effects performance wise its made, or not..?
The after market panel filters offer less resistance to the air going into the engine, whereas the standard panel filters are more resistant to air traveling through them.
I think you can pick up a good panel filter (k&n, pipercross etc) for between £30-40.
A standard airfilter can be anything from £5-10.
The thing with an aftermarket one is it never needs replacing, just cleaning every so many thousand miles.
I'd also be interested in how good the filtration is on these filters. On my old car I got a cheapy Ripspeed cone filter, the material looks identical to K&N filters. I'm a bit concerned about using it on my new car, would it be worth getting an official K&N filter or would it be ok to keep using my cheapy Ripspeed one? Just curious coz it was half the price of a K&N, but is the filtration not as good or what?
I don't think they are tbh. You may aswell not use an airfilter with them cheap ripspeed or maxpower ones.
Having had both a cone filter and a panel filter I would definately advise getting a k&n or pipercross panelfilter and utilising the manufacturers airbox.
There is no better way to get lots of cold filtered air into your engine.
filtration efficiency on a wire mesh-type filter will depend on what oil you use as well, i use brake cleaner to loosen most of the crud, then hot soapy water, dry it and pour automatic transission oil over it, after leaving it to drain while i crack on with the rest of the service, i give it a wipe down and spray a bit of wd40 over it, jobs a good'un.