The regression has nothing to do with the YM2612 improvements, all of which were tested in MESS originally and based directly on tests run on actual MD/Genesis hardware. The regression is purely a result of the u7 changes.
Anyway, you can "bring attention to" it all you want - the code is Haze's to fix and for hopefully obvious reasons nobody else wishes to touch it.
PS: "soundcpu" and "maincpu" are the names of the Z80 and 68000. There is an implicit mapping.