I fitted an extended heel shifter and dropped both the toe and heel shifter down so the top of each sits just the width of the rubber above the foot plate. Size 12 boot, no problem.
At least everyone here says to install the spacer. The Harley dealer in Tampa sent me on a demo ride on a RK that had the heel shifter taken off....when I got back I asked what was wrong with the transmission, it shifted like crap.....after careful inspection by the salesman I noticed the spacer missing and also noticed the smooth spot on the engine where the shifter had rubbed during the previous owners reign on the bike.(yes it was a used bike) You would have thunk they would have caught that before putting it on the floor.
I like the heel shifter but took it off because every time you hit a bump while shifting you jump two gears. I took mine off and pulled the front shifter outward and put a Kuryakyn spacer on. It's much better and easier shifting.
I just removed the heel shifter a few weeks ago. I can't believe I didn't do it 3 years ago! I slid the toe shifter in and used the Harley end cap. I think it was around $9.00. Very nice without the heel shifter there.