Paddy, you're heel-and-toe practice sounds good. Do it on the street all the time, once you've developed confidence in your technique. By doing it all the time, you'll put the ball of your foot in the perfect positions all the time and it won't be a change in the heat of battle. On the street be smooth.
The most likely heel-and-toe downshift in AX is 3d to 2d, but the braking will be much harder than you should be doing on the street. A 3d gear straight is fast for AX and almost certainly will be followed by a relatively slow corner. Getting to where you can threshhold brake while heel-and-toeing will take a bit of practice.