Can you put the mix on a switch? You're mixing some down and right aileron with rudder input, right? Maybe there isn't enough of an effect to cause a problem when you're slipping or crabbing. Check it out and let us know...

I'm no aerodynamics expert, but I've always thought that rudder inputs which produced roll or pitch coupling in knife-edge also had the same effect at any attitude. So IMO the mix will be useful even when you're not doing KE.