I have mix added as well and so far for regular harriers It does not act against me. It still reacts as it should so will be leaving it alone. I am sure that if you have a good bit amount of mix added you will see it make the plane react in ways that you would not prefer.

I keep my mix on full time and if anything I think it helps the plane all around for 3D. For one doing 3D there is a lot of rudder corrections to maintain level flight or high alpha flight. For example, doing rolling harriers require the use of rudder. Say your plane pulls hard when doing a knife edge to the canopy. Well in a rolling harrier when plane rolls over to knife edge position it will tug the plane the same as it does doing just a normal knife edge. So while not only will you be trying to correct with rudder for RH but now you added an extra correction to the RH that was just to keep the plane flying straight.

Okay so say now that you have added mix for knife edge and it tracks straight no pull. Now in RH when rudder correction is added your plane will keep a nice straight line and not wonder off to canopy side when rudder is added. So right there you eliminated an extra correction and made the RH that much easier. Now you can really focus on doing the RH itself and not have to fight a tug to canopy. You can now think of where you want to RH your plane to, figure eights, circles and so on.

The mixing helps a lot with pretty much all the 3D moves. Even if your flying slow in a normal inverted harrier around the runway. Say you want to roll back over to right side up position. You roll over and use rudder to keep the nose up until right side up. Right there if no mixing as added and your plane pulled hard, you would have a weird jerky looking roll over to normal flight. With the mixing it would be just a simple rudder to elevator and continue the harrier, no pull to side no extra corrections. It would roll over and still track smooth and without the jerky moves.