I understand your reasoning but disagree that the worst-case effects of this type of change warrants a 7.9. But if we get a 7.9 soon I'd be more positive; maybe we can develop there and backport if it turns out to be an improvement we want to distribute asap.
Regarding the identifier cache, is the cost for a cache miss proportional to the number of identifiers in an object? That could be a possible reason for excluding them.
