haha, sorry mate, It was out of my text window so I forgot I ment to reply :P
I did see an S4 that had the letters of his number plate cut out, that looked amazing. From a distance it looked like a standard plate, but up close, the characters were infact holes! how much air flow you'd get through that I'm not sure though.
For your car, I'd probably think a few decent sized holes under the plate, along with corresponding holes in the crash bar would be the best you can do, then remove the plate for trackdays.
Like you say though, if you're getting enough air flow into the core anyway, is it needed?