Yeah, heal/toe can be very fast, very clean. I have watched video of Virgil's feet and believe he is doing a heal/toe, not a slide technique, but it is hard to tell exactly what he is doing. If it is heal toe, he is not lifting his heal very much, which is what I would expect. When I do fast rolls using the technique, you can hardly tell I'm raising my heal, but I am. It is easy  to double the time from 16th to 32nd using heal toe. What has taken more work for me is to mix up single strokes with the doubles.