I am about an 8.25D on the brannock and went with an 8.5D. They are super comfy and light but as you noticed have a little bit of room by the heels. I do get a slight amount of heel slip but nothing crazy. I would say go TTS on them if you are considering. I have a pair of loafers from GH Bass that are made by Rancourt in 8D and they were too tight so I decided to size up on this pair.
Totally, I am blown away by the lightness and comfort. As many have mentioned before though, the Rancourt leather soles are pretty terrible and are wearing down quick compared to AE or ALden leather soles.