Not sure you would notice much...
Given Clift = 1/2pV2s... , the only variable here is V2... speed.
Approximately... at 200 MPH in the GT430, we get 200Kg (196kg) downforce, so at 150 mph we get 100 kg and 100mph we get 25 kg.
Average speed in the corners would be say 100 mph... so the winged Evora GT would give us 25kg and lets say, the non winged/aero Evora, 0kg.
I doubt 25kg over 2 rear wheels etc would make any difference... well no more than having a 100kg pax in the pax seat!
At the Indy '500 Oval' however, I am sure there would be a small difference if you could maintain 150 in the corners to get that 100kg!
It's good sales/bar talk however! 😄