Simple solution. (Simple enough that i thought it went without saying.)
Let the "spec" racers run thier races. Offer flags to lock out changes on different tabs.
there are other adjustments you can make that make a difference
Timing for one. in this case the adjustment would be simplified (you can only adjust within the range that wont damage the engine) within this range it's possible to adjust your powerband. at one end you have a nice street car with a wide, but not too powerful powercurve that is very very forgiving vs the other end which makes a lot of power over a very narrow band and doesnt forgive at all (this example is a very extreme, you MIGHT gain 10 hp on most engines)
this is for the guys who want to go big. touring cars and GT cars (it might even allow those GTR cars to all run evenly by adjusting powerband to the course)
and i dont have anything agains Spec racers. Spec Miata class in the SCCA is one of the most competitve classes in the world. But on the other side of the coin, the Speed GT class isnt easy either.