This thread suggests they grow faster than they do in my experience...
.5" per month on average from 1.5" to 6" is more in line with my experience...
Keep in mind many many things influence growth rates from diet, to water quality, to stress, to genetics...
In my experience yes Dempseys grow a bit faster than GTs (Saums) but the "typical" adult size of a Dempsey is slightly smaller than the "typical" adult size of a GT (Saum).
Sexing prior to sexual maturity is not reliable, as markings can and often do change... sexual maturity and therefore reliable sexing generally occurs between 3~4"...