This is the actual side area the duct is enclosed all the way back to just about the stock COG.

As far as I know everybody is adding a lot to the rudder, many I have seen use large counterbalances also. I had hoped that by adding to the side area ahead of the COG would be enough to improve the knife edge. Perhaps I should add a lot more rudder than I have already.

AS far as the snap with elevator/rudder I do not know I have not flown one myself, yet.
Perhaps Tall Paul could answer that question he has had several by the looks of it...

It is a tapered wing that is why I am concerned. Stock it has a real fast roll, although I have cut down the ailerons so it may not be as responsive in roll. Although I am setting up the ailerons on pull/pull so I can mount the servos 8 inches further inboard than stock to make the wing tips lighter, and keeping the most mass close in to the roll axis of the plane.

perhaps this view will help