Received my Marlow wingtips today as well. Experiencing the same as you gsgleason. The heel does indeed create a pivot point. I assume over time the heel will flatten and this will become less noticeable. I am on the fence about whether or not to return these, as I have a very small bit of heel slip on right, and a bit more on the left. As a comparison, AE 5 last fits me almost perfect after break in, but it is an overall narrower last. The Marlows off the bat are more comfortable, more room across the vamp, not excessively so. The length going TTS in my US size is just right, I do not think I could size down a half size without having issues in the toe box.
I guess the ultimate question is, will the slight amount of heel slip disappear after break in? Any thoughts would be appreciated. The sizing on these things is really YMMV. Too bad the local RL store didn't have my sizes in stock to try on to compare.
I assume both these issues would get better as the sole breaks in over time. It's quite stiff at first but should get looser eventually.