Check with the Florida groups who tried this unsuccessfully last summer. I believe that they are going to the USVTA rules this summer because of the tire wear, large speed discrepancies and an overall lack of turnout because of the speeds.
If I'm not mistaken, the HPI Challenge limits racers to ONLY HPI cars, so you are cutting your numbers down a LOT right there. Not enough to base the whole season on that one race, IMO.
Regardless of the size of the track, you shouldn't increase the speeds to compensate. Add chicanes or other technical areas to larger tracks to keep things interesting, but making T/A cars faster just makes them classes that already exist (and are becoming more poorly populated).