With the lens on, the grip is not going to take up any real space. With size increase 3 to 4mm in width and length is not going to be a problem. Just hope that the thickness does not increase . Also hope that that Canon will put a bigger battery inside the grip.
Maybe you are right. Probably will need to wait to see it in the flesh to see how much of an increase it is.
Depending on the price of the M3 a used A7 may be a better purchase for increase image quality.