That sounds pretty crappy man... I wish I could help you out but I don't ride flows. It's good to hear about this problem though, cause I've been thinking about getting a pair. I've heard mixed reviews from people, and I ride park, so I need a sturdy binding.
Heels sliding up in the binding would be a BIG problem, as you've said. I guess I'd have to ride em to know for sure, but I think I'd rather live with the extra 15 seconds it takes to strap in and know my feet aren't going anywhere.