Gene,

I had that same mount in my Focus 2 with the /F3A motor worked great... for all of 60 flights, but there were no issues at all as far as twisting or flexing. Mount is very rigid and stiff. I replaced the front plate with a carbon/plywood laminate to reduce weight, and ground the heck out of the rear plate to save a few grams (waste of effort, saved less than a half ounce). My Dad is has that mount on his Focus too, works really well and he has probably 70 flights or so on it.

Because the motor sticks out farther though, you want to make sure the firewall that the mount is connected to is secure. We reinforced the firewall with light aluminum angle pieces to make sure that it was stiff enough.

Tom