FWIW, my opinion on the matter is that mirror slap is probably only a minor player, depending upon the situation.
I shoot an M645; all in all, it is an awkward camera to hold and control in the best of conditions. It is very heavy and not ergonomic at all. Without my L-grip, handholding at below 1/60 is simply not on. I figure that the grip, or lack thereof, has more to do with shake at low speeds than anything else.
If it's that bad for an M645 at low speeds, I cannot imagine a P67!!!