Well, having a dominant autoflower gene would make retention of the autoflowering trait during selection a lot easier, that's for sure.
It could take fewer generations, I think.
At this point, though, many if not most of the commercial autoflowers are already 8-10-15 or even more generations bred away from the original hemp-like ruderalis plants. Apart from the autoflowering trait itself, these don't retain much, if any other ruderalis genetics at all.
Remember, with appropriate selective breeding, you can reduce the percentage of autoflowering genes by 50% in every other generation, just by backcrossing to a photoperiod plant. After ten generatons of crosses and back-crosses (ie just under three years of work), you can turn a line from 50% ruderalis genetics to less than 2%, and that's without any selection at all. This also assumes you're starting with a pure ruderalis plant. If, instead, you're starting with an already worked line, you're probably starting with something that has less than 10% ruderalis genes to begin with.
As a related issue, I think its an open question whether or not just having an autoflowering gene in a plant affects its potential potency.
Emprically, there are any number of fairly potent auto-flowering strains out there now (eg 60 day Wonder is supposed to be pretty potent, as is Paki Kush ryder). So its certainly possible to have strong auto plants.
But if this is true, you're just never going to have autoflowering plants as good as the best regular photoperiod ones, even if you were to exclude 99.999% of all Ruderalis genes from your line.