I have the same problem on an Ultimate, at low rates I can mix out pitch couple with rudder, but at high rates I need a different mix. So I have tried a multipoint mix (futaba ff9 5 point mix) which works in some situations like knife edge, but its completely wrong for stall turns.

The solution is to have the mix on a switch, so you just use it when you need it. Ideally if IMAC is your thing, get an airplane with a symetrical setup where rudder is pure.