It hasn't taken me long to get totally used to the heel shift. I've only been riding for a couple of seasons, started on an XL. So, I don't have 30 years experience like some others here of just using the toe shift. But recently my heel shifter fell off of my FLHRC, and even the short ride to the dealer to get it put back on, I kept trying to stomp the heel shift to upshift. It's just become engrained in me now, and I feel it to be much more fluid than just having a toe shift.
I think the key is to set the shifter as low as possible. I just slide my foot back (barely even having to lift my heel) to upshift. I would agree that anyone with a size 12 foot or bigger may feel a bit cramped.
ride safe
Drew