My opinion for what its worth:

THe servo motor is strong enough but the gear train is still plastic. Thats not the point. Its all about handling flutter. Its not strong enough to handle any flutter at speed. You should be running metal servos on a jet flight surface. or HBs. I like the 821 servo, I run it on my 3d birds and everything else, but not on a jet. When you pull negative G, you are also fighting the venturi effect of the exhaust flow. If you dont believe me, go full elevator down and hit your smoke and watch the turbulence at part throttle, to simulate wind flow. Remember the airfoil is semi symetric so it will always be harder to push out.

I also used 1-2 degree downthrust. I think its about right.