Supposedly the 5-last runs long and narrow, while the 8-last fits more TTS. I think it is typical for AE associates to recommend sizing down a half but up one width for the 5-last.
Also, I suppose the 9.5E is going to be shorter than the 10D in the same last. Since the 5-last is long, and my arch-length is longer than my overall length (short toes I guess), this made sense for me.
Yes, and the fit will vary depending on the type of leather (cordovan/calf/suede/etc.). But even on the same last, there's not much difference between a 9.5E & a 10D overall. Though the change may be enough in one area of the shoe to cramp the toe box or loosen up the heel. So, it's certainly worth distinguishing between the two sizes.