Left hand grips are evil things.
They are sold with the promise that they make life easier, while they do just the opposite. You do not want to hold a heavy camera on the end of an arm, on the end of a lever, constantly opposing gravity by twisting your wrist. The only way to use such a grip and stil be able to actually hold your camera (as opposed to have it dangle unusably) is to use your other hand to support it.
Absolutely terrible. They ought to hang the person who invented that such that he can save his life using a left hand grip...
MF cameras are best held in your hand.
Best MF cameras are those that are designed around that, i.e. only need the other, free, hand to operate the important bits. Most do: one hand required to hold the thing and operates the release, the other free to do anything else. Some don't: they have the controls on both sides of the camera, requiring changing your grip to operate them all.
Your Mamiya is one of the better designed cameras.
A motor grip will, in my opinion, not do much to improve holdability.