Idle isn't the problem. If you're sitting there, the engine isn't generating much for heat so fans connected to a radiator positioned almost anywhere will be enough. When you're at the track and on it for 20 minutes, you've got more than enough heat and the fans may not keep up.
Ideally, there's very little air under the car. A properly set up chassis will create a vacuum under the car, and even on an improperly set up car it'll be fairly low pressure so there won't be much flow to pick up and stuff through a radiator there. If you have any decent air under the car, it'll produce lift and all the horsepower in the world won't get you around the track as fast as a well tuned stock MKI Scirocco.
Experience tells me air is gonna have to come from somewhere else to keep up with a VR6, more if it's got a hair drier.