I recently spent an afternoon with a Yamaha Z (mine), and Selmer Series II, III and a Ref. I did not play several of each, so this will not answer your question directly. However, I played one of each and every one of them was an excellent sax with no particular variance in quality. If Selmer can do that with three each from the different series, I would have no reason to believe that there would be a wide variance within any one series particularly.
This is not to say, of course, that each within the same series will be exactly alike, but I would suspect the difference would not be great either. When I tested altos last winter I played several of each series and, while I believe the one I selected had something extra over the others, the quality was pretty uniform. I would also add that they were all set up before I played them.
Regarding the differences between Selmer models, since I prefer the Serie II alto over the other Selmers, I expected to do the same with tenors. Not so, though. I did a blind test and, while the Ref and III were pretty close, the II just wasn't in the same category. (Disclaimer - keep in mind I only tested one each.) BTAIM, I would only be using the tenor for jazz and pop and liked the other two much better. Regarding the differences between the Ref and III, I could've lived with either. I would have had to spend a lot more time A/Bing both to make a decision. If I couldn't make one, I would probably go for whichever had a better price and if the shop would throw in a couple of freebies, like a case of something.
BTW, even though they were all set up beforehand, my experience playing new Selmer altos and tenors this past year makes me think that Selmer's quality control has improved.
FWIW, I decided that, for what I was getting out of all these saxes, that there was not that much difference between the Yamaha Z and the Ref and III to justify replacing my Yamaha for a Selmer. Also, LateNightSax, an acquaintance, Dino Govoni (go to his web site if the name doesn't ring a bell) was looking for a back up for his Mark VI and settled on a Yamaha Z UL, not another Selmer. Furthermore, he has found himself playing that horn as much, if not more, than his VI, so you this might be something you might consider and (not your primary goal I'm sure, but good icing on the cake) save yourself a good chunk of money in the process.