With good tires, grip should not really be an issue... Not unless he's doing 100mph on a twisty mountain road.
If you get RS4 body panels, you'll have all the downforce you'll ever need.
Unfortunately, the RS4 is only available as a station wagon right now, so you'd have a hard time finding 1/4 panels. The air dam and front fenders would be no problem though.
It's the same body kit, fender flares, side skirts, rear bumper cap, but it's a station wagon...