If he does it right, it will be less dusty inside. Our rally car has a roof scoop. With the roof scoop closed, the dust comes in through the door seams (I sealed a bunch of holes in the floor right after we got it). With it open, the roof air pushes through and out the gaps around the doors, eliminating the dust. You have to have the windows rolled up, though. See #4.
And if you notice photos from rallyxes, you'll see that the hood is almost always out of the dust, while the fenders/wheel areas are enveloped in thick clouds of dust. And without fender liners, the filter gets filled up pretty quick, and then dirt gets inside the engine and kills your pistons.
He really needs the roof scoop. I stopped riding with him after a couple of times, because I couldn't breathe afterward. It's really, really bad in there.