Keeping records will tell you even more. Just because certain breeds are "known" for certain qualities doesn't necessarily mean your birds fit that mold. They may have come from a different strain of that breed or may be mixed or not "true" in some manner.
Keep records of how much each bird eats and how many eggs they lay. It can be difficult to accurately measure just how much each bird eats, but two observations will help you get a ballpark idea:
1) When they go to roost at night and are getting a little sleepy, go into the coop and feel each bird's crop and see how full it is. Over time, you will have a better idea of those who are a bigger eaters than others.
2) Observe how much time they spend at the feeder. To get a true sense, you'd need to watch a few days worth of video pointed at the feeder and see who is eating more. But realistically, just spend some time with them to get a rough idea.
You've already noticed that your RIR are crowding out other birds. RIR are known to be more aggressive than many breeds, so it could simply be their aggressiveness. However, if you notice that the RIR are spending more time at the feeder than any other birds, have fat full crops at bedtime and the RIR hen is laying fewer eggs than your other hens, then you have your answer that your RIR hen is probably not as feed efficient as the others. You should not breed her, because you will probably be passing along her less-than-stellar feed-to-egg trait.