I agree with the dihedral as well. the tailerons just will not have enough leverage for all of that span and area. Not only that, it is a constant chord wing which is not as easily rolled as the tapered variety of the little jets. Just adding dihedral might do it and then maybe mixing out most of the aileron on the stab. Using Rudder to mostly turn the aircraft. If it were me, I would put the rudder on the aileron stick, with slight amounts of taileron mixed with quantities figured through testing.
Also, I would lower the incidence of the wing, assuming the Stabilator was set properly in the first place. Also look at down thrust to keep from climbing. Maybe 2-3 degrees.