I wasn't aware that Flightaware already has the information about actual mileage freely available. Thanks for alerting me to it! That means that the technology is already in place. As for data points, here are a few:

UA264 SFO-SAN MONDAY 07-DEC-2020 Arrived 17 minutes early. (Not too shabby!)
Actual: 510 mi (Planned: 511 mi/Direct: 448 mi)
Actual is 13.8 percent above great circle distance. But since the generous 500-mile minimum would kick in, the difference is only 2 percent. Again, that's 2 percent with the help of the 500-mile minimum.

UA447 SFO-DEN SUNDAY 06-DEC-2020 Arrived 3 minutes early.
Actual: 1,054 mi (Planned: 1,032 mi/Direct: 966 mi)
Actual is 9.1 percent above great circle distance.

UA277 SFO-SEA SUNDAY 06-DEC-2020 Arrived 2 minutes late at the gate.
Actual: 716 mi (Planned: 702 mi/Direct: 680 mi)
Actual is 5.3 percent above great circle distance.

I think you'll see the biggest impact of the great circle policy on medium range flights that are not eligible for the 500-mile minimum but where the deviations from actually flying great circle routes due to take-off and landing are still impactful.