If you haven't tried the Burton TWC (Shaun white) boots, DO SO. I have a narrow heel as well. They provide the mid flex you're looking for and I feel the upper support is great. I just bought a new pair of them on zappos for ~140.
I tried EVERYTHING on at the stores at the beginning of this season and my heel slid around in every boot. Then I figured, I might as well see if I can get the same shaun whites that I had for the last 3 or 4 seasons. Zappos had them at a good price and the newer model fits and works exactly the same as the older one.
LOVE these boots