A thought on leaving some venting for the brakes, obviously the outer part of the wheel is spinning fastest, but vented brake rotors are also throwing air outward against the rim, so if you blank off the center but leave a few slots around the rim, then brake air can still escape. If it's totally blanked off, it'll recirculate back through the rotors again which is no bueno.
I also noticed that new Porsches are coming with "air curtains" ahead of the front wheels just like BMWs have had for years, I wonder how effective they are at reducing drag from the wheels. Apparently the Land Rover Discovery lost something like 0.003 Cd thanks to them.