Ok, so I got the mediums again. And by adjusting the heelcup as john doe pointed out, my boot does fit well on the board now.
So, I'm using the 7.5 Salomon boots in the Medium GT binding. And I'm STILL getting a lot of heel lift
On top of that, the smaller toe straps on the medium seem to result in the loss of the ability to get my foot in place without re-adjusting. The toe strap just wants to go too low and I have to loosen it/pull it up/re-tighten every time. Although that is kind of disappointing because it defeats the purpose of speed entry, at least it's usable. The heel slip problem however is not usable. I haven't measured yet, but I feel it coming up, and usually even hear it as the back of my boot slides up and down the highback and squeaks.
Are there any adjustments to help with that? I have my highback lean set to the second line from the bottom. I could go a little further but that is starting to get aggressive.