I really don't understand the new limitation, because you're already giving people fairly significant methods of gaining advantage in online games with the settings that are currently open.
Even without changing tire grip or downforce settings, you can drastically improve the handling of any car in the game by changing ride height, suspension rate, "primacy", gearing, and transmission type.
A car can be made to accelerate faster and handle far better than a stock vehicle with the settings you already have open.
Example, take two identical cars, both RWD 5-speed manual shifting cars, modify just one of them to be 4WD, have 7 gears and have sequential shifting, shorten the gear ratios, lower the ride height and stiffen the suspension.
Even though they both have the same horsepower and the same top speed setting, the modified car is going to get off the line faster and handle better thanks to the stiffer suspension and 4WD. It will accelerate faster thanks to the shortened gearing, sequential shifting, and two more gears.
Limiting the top speed at this point is like trying to put a cork in the top of a wine bottle that's got a bunch of holes drilled in the side.