I use a filter on the MAF, pretty much just for throttle response, and sound. On newer cars, the airboxes are usually designed pretty well and aren't a restriction. At least for performance cars. Hot air intakes really don't matter on a turbo car, since the drag of long piping to the turbo pretty much negates the gain from cooler air. Hell the turbo can heat the air up to 300 degrees under high boost. So that 15-30 degree difference to the pre-pressurised air doesn't really matter.
On a NA engine, it can help though.
However while cruising usually the bay isn't nearly as hot as when you are sitting in a parking lot showing off the car.
Big Montana wrote:What cars should they not go into or is basically every motor VW makes OK?
From my readings on a few other sites, it seems anything from VW and Audi from 98-99ish (MK4 and B5 chasis) up seems to have very little if any effect on power. They may yield a couple up top, but nothing noticeable to the butt-dyno. I know I can't tell a difference in power from the stock airbox to the K&N I put right on the MAF. I can feel a slight improvement on immediate throttle response, but that could probably have more to do with the factory filter than airflow.