I know I'm past 5", but don't know how far. Mine will hold a 45 degree up line inverted without any push on the elevator stick. I had both my 1100 ma packs just in front of the rudder servo(pull-pull), and it got a bit twitchy for me, so I moved one of them(4 oz.) up over the landing gear block and it's much better.

Try moving a battery pack towards the rear 3" or 4" and go fly it. I think you would really have to get back in the 6" range before the edge would get uncontrollable.......

I'll try to remember to check the CG next time I have it together.......