Dear Sir/Madam,
Is there an empirical data available on what the loads (lbs or Newton) are on a typical splitter for an average passenger car at various speeds?....or is there an easy way to calculate/estimate the force  for a given splitter size/area ?

Thanks in advance
kindest regards

I believe modern F1 cars are up around 8 lat G's through the high speed corners. 

But that said, I cant think of many corners in the F1 schedule that are taken at over 300km/h, which the top Time Attack cars are doing. So they could actually be in close territory.

Keep in mind that those classes have very little in common with a passenger car other than their exterior body shape, so it isnt totally surprising. Open class is really your more standard high aero passenger race car, and they will be managing around 3G's lat.