I'm still more interested in averages, rather than maximum speed. My GPS may show that I hit a top speed of 75 mph, but that I averaged 53 mph while moving. A top speed may very well be an aberration, however the average moving speed could be more of a tell-tale. If you just took a look at the top speed indicated, said "OMG!!!" and fired the guy, it might not be quite right. If the average speed was too high though, then you've got a better case. Anybody--- even you-- can get a heavy foot on occasion which would give a high top speed.
My unit shows total miles, top speed, average moving, average overall during the entire trip including stops, moving time, stop time and total time. All of these together can tell whether the lead-foot was a single incident or if it was a habitual way of driving. My Chevy van is fast if I choose to do it, but it's not an Indy car and shouldn't be treated that way.