The ECU rule is a red-herring. My understanding is that the rule was changed in two phases.

Second, and most liberal change, free ECUs. The motivation for this was equity and to lessen the cost of the arms race. There was a set of cars that could cram an after-market ECU into the stock housing a/o custom chip installed in the stock housing. The cost was prohibitive for most, but for the big buck guy, within budget. Given the relaxation of the original standard, it made sense.

First, and least liberal change, was the stock housing rule. As I recall, this was not done to allow allowances a/o performance improvement. It was done because the stock ECU rule was becoming/had become unenforceable. Consequently, dropping the stock ECU rule made sense.

Now alternate final drives... there you have something.