Are the mounts tightened on the photos you are showing?? Just looked loose to me.
Only plane I have the Dubro mount on right now is A very old Bridi Kaos 120. I removed the engine and mount from an Extra I sold. This is A small 1.40 22cc gasser from RC Showcase, you can see where I had drilled/cut out the side of the mount for throttle or choke clearence. With this engine in both planes there was no flex or any problems. After I get the mount snugged down onto the fire wall I do use A torque wrench just to get it all equal. Can't remember the torque right now but I do have it writen down. It isn't sinched way down but there is some squish in the rubber.