I can't think of any reason for there being a difference. I think your inputs are still done on your PS3; they aren't sent to the PD servers and constructed there. If you're playing with a dodgy connection and you're lagging, other cars will jump around but your own feels exactly the same as it always does, so for the most part the control of your car isn't reliant on any data transfer between your PS3 and their servers. The only parameters that could influence your car that's reliant on data transfer are the aerodynamic influences of other cars and their positions, which of course tells your system that you're in a slipstream/dirty air/colliding with another car. If you suffer from lag then using wired over wireless might give you a slightly smoother connection. You might get slightly more game data through to your PS3 outlining those parameters which might result in you picking up a slipstream a couple of frames earlier I guess, but the difference would be unnoticeable.
I've played with both connection types plenty of times, and I've never noticed any lap time differences, I'm afraid. The only real difference is that wired connections are more stable, and perhaps marginally faster.
