Jack, I would be concerned about adding an extra 4oz. The Aeropoxy will hold that firewall just fine. In fact, the soft mount is not going to do much for you except add some weight. It is a good product but I don't think you have the weight to spare. I know a friend flying the Impact with the Axi F3A and it was flying at 11 1/2lbs so it is tough to make weight with this combo. Although even at 11 1/2lbs it flies fantastic.