How much difference in time could there be? (Not arguing too much, just wondering out loud :))
My car has a huge front overhang with a low splitter. IIRC, it is about 48" from the axle centerline to the tip of the splitter, which is a huge, low overhang compared to almost every other car. The front tire is 25.5 diameter but you wouldn't get the radius as an advantage, but I would WAG that there might be 30" difference in launch point from front of car versus center of tire.
It also accelerates faster than the average car when it has grip. Hitting 1.0+g accel on launch is pretty easy on concrete.
The only cars I can think of that would have more overhang and more acceleration off the line would be the BM cars with those low forward wings.
I can't put my brain around the math to figure the exactly number, but I can't see where 3 feet of acceleration is even measurable with what we do? I know it would be measurable on a drag strip, but we a) don't have nearly those kind of launch conditions and b) almost always have a hard turn or other element right after the start to negate acceleration advantages.
I dunno, not arguing, just rambling out loud.
Oh, another reason there is not a rule in the book is that the book assumes national event, running by class, and no PAX. As long as the starter was consistent during his heat, no competitor would have an advantage over another.