In theory, you are correct. But how much would you gain? IMHO the wheel well turbulence, denied entance to the wake, would spill outward and at the rear edge of the end-plate would be mangled (flattened) and mixed in there. Simpler and it doesn't compromise the difusser.
Then there's all that salt and chunks of tread that won't build up.
Re: The rules—that was the only car I saw that had fences that weren't longitudinal, which is suposed to be a requirement. But it passed tech inspection.
kach22i -- ???