One of the sticky points on some of these cars is that they also have stock HP numbers outside the typical envelope for the class (Civic Si is certainly one in ITA, but there are others to be looked at). ...
It may be the case that, more than we really understand, the differences between classes are about total power, rather than power-to-weight ratios. I'm basing that thinking on some of what we've seen in STL recently. If it's the case, it is of course going to be track-specific to some degree.

I personally think dual classing is a little silly, not believing that - ASSUMING the car is run to the same, correct process in both classes - the difference between being in A or being in S is the real decision-maker on whether to go racing, or what car to run.

As Jake G has so accurately explained, if the process lets the 1st gen RX7 down in A, it will do the same in B.

I suppose that different priorities (ease of construction vs. ease on parts) would potentially motivate two entrants to choose different options but at the end of the day, it's just a complication without much payoff, I don't think.

K