Dude, before you start changing the volume of the fit the boot by messing with heel lifts, please remove the spoiler on the back of the cuff. This is 100% the purpose of the spoiler- to take up room for skinny calves and be removed for big calves.
Spoilers don't add stability, they simply add volume. When you take it off your boot, just make sure that the screws that were holding it on are now not too long without the spoiler in place. You don't want the screws digging into your calf.