Yes, the purpose of the revo mix is to essentially give the gyro a headstart so that you don't get heavy swingouts during throttle changes. Without a revo mix, the rate gyro would still work (albeit it would drift alot) if you throttle up and down VERY slowly (assuming your tail linkages are setup properly). But if you punch or kill the throttle, the helicopters tail will basically swing out very sharply because the gyro can't respond quick enough to counteract the ammount of torque being placed on the tail so quickly. Remember, a rate gyro only corrects when it senses movement, a heading hold will correct wether there is movement or not, so it doesn't have to play catch up like a rate gyro does, hence theres no need for a revo mix on a heading hold gyro.

This is my general understanding of how the two different gyros work. Some of my operational knowledge here may not be 100% right. I'm sure someone else will chime in if i've said something stupid.