I have one thing to add. Because the throttle pedal is hinged on the floor it is much easier to "heel-toe" when your heel is on the floor.
Up until this car, I only had manual transmissions in Japanese cars (MR2, Miata,...) which hinge their throttle pedals from the top. So, I was used to having my heel off of the floor when heel-toeing and had no problem this way. In fact, this was more "left-side, right-side" rather than real "heel-toe".
I had an E46 M3 but that was with the SMG transmission so no heel-toe there. But with the E92 M3 I have a manual and was really struggling to make heel-toe work until I figured out that you really need to keep your heel on the floor at the base of the throttle pedal.
To brake, pivot your right foot over to the brake using your heel as the pivot. You can still leave a big hunk of the outside part of your right foot on the throttle pedal to operate the throttle as you're braking with the ball of your foot.
I hope this helps.
I've been assimilated. 6MT/ZPP/18"/PS extended.