I did compile a fresh 7.9 and then copied all the bench files back to an old 7.8 and ran both to compare. Although most things were noticeably improved there are a handful of significant regressions:
I have not investigated call_out or compile (I will tomorrow), but as for the tag removal tests:
The reason was the opcode LOCAL + LOCAL. It did not do the trick where it first free:d the old value before calculating the new one, so the various string/array += optimizations were not triggered.
The tag removal tests build the string (basically the input but will all tags removed) using += (so it is, in a way, as much of a string addition as tag removal test).
A fix has been commited (I also did the same fix for ++ and --, which might be considered overkill, since they seldom operate on non-numbers, but..)
I also made string allocation significantly faster, which further speed up the test.