i have done some testing on track with a few bits and i can tell you the biggest differance in time was a deep front spoiler and splitter, just under 8 seconds at prescott which is under a mile long
didn't feel a lot different. the rear wing i run makes approx 80kg of downforce at 90mph and makes a huge difference to how the car feels at speed but not a lot in time, just feels a lot more stable.
if you want to make a big differance but not have it on show then try and make the under side as flat and smooth as poss with a slight front rear rake. front lower. vented backs to all wings makes a differance.
what ever you do make it strong, made the mistake of making the front splitter mounts out of 1.5mm alloy plate, at 100mph it bent
now you can stand on the splitter.
cheers rob