Not scientifically, but empirically: shooting the 300 lens on a monopod, with MLU and getting some pretty clean shots. With mirror - less so.
So too, MLU has come in handy with normal lenses (60 or 80 mm) and slow shutter speeds, hand held. I typically take 3 quick shots with MLU, and usually one of these is sharp and the others aren't. At slow speeds hand held, the ones with the mirror in action can be a bit soft, although the dampening is quite good, and better than the 6008.
So while it seems that with the Hy6 the mirror dampening allows you to pick up about two stops on the 6008 (for handheld), the next jump benefits from MLU and leaf shutters.