You can tolerate some fairly large holes and several holes in the firewall if you back them up on both sides with large steel fender washers. I suppose if you get to the point where the firewall looks like a piece of Swiss Cheese, maybe you've gone too far though! The pieces of dowel you install stop the firewall from compressing when you tighten the bolts but do little for strength otherwise. I've seen three of four sets of mount drillings with no ill effects.