Fact is cone air filters can help performance, virtually none have any downsides.
Giving better performance is NOT the same as passing a MOT, the difference is the speed of the car, at mot time it is stood still sucking in all that nice hot air, this WILL RAISE THE emissions simple as that, going along the road the hot air is draughted out the engine bay area and not ingested, unless it is one of the really badly designed cone filters.
Put a cone filter on a cossie for example and take emissions readings without and with the fan on, I can guarantee you having seen this loads the emissions(CO) will rise, due to the hotter air containing less oxygen.
My own cossie does this, so I just wait for the fan to come on then stop, then take readings.
As I have a temperature probe in my air filter end(so not in direct flow)I know the temp of the air filter air, and also inlet air in plenum(another probe), having used this(2 probes) with both air box and cone filter(with NO heat deflector), the perceived "hot" area on a cossie at least behind the drivers light where the airbox/cone filter is is NOT a hot area, remaining for 99% of the time within around 2C of ambient, from 30mph upwards.
Just my observations, both probes were zeroed on my temp gauge, down to less than 1/10C, and the gauge reads down to 1/10C.
tabetha
tabetha