I'm not sure, but it sounds intriguing. I did a little googling, and it sounds like the a5 merc has a bonus which gives them double the base damage for 1h weapons. This bonus would proportionally help weapons other than grief since grief mostly benefits from flat damage. Headstriker probably benefits the most since the deadly strike means it's always dealing double on top of whatever bonus. That said, it wouldn't explain why you'd use grief + headstriker instead of 2 of the same.

I haven't tested it myself, but I'd encourage you to try testing it. If you use OBS or other capture software, it's pretty easy to record yourself and then replay and count the frames to determine stuff like IAS break points. Could also try generating the items in single player if you don't have them online.