Okay I purchased this to update an R9000 derailleur. The derailleur cable routing was a little long with the R9000. With the R9000 I needed a wolf tooth extension to use 11-32 gears on my Zwift trainer. I have installed the R9100 and it is has the wolf tooth extension built in and it has a longer cage than the R9000. I have put it through the wringer on zwift with CVR world cups. I can use the full 11-32 including the 39-11 combo which I could not with the R9000. I will never use the 39-11 combo but the longer cage helps out a lot. What I have noticed is the shifting is considerably lighter and more precise that with the R9000. Also when climbing a steep short hill and switching to 53-32 momentarily; I don't get the grinding with the R9100, that I did with the R9000. The pulleys seem to be more smooth and the entire drivetrain is more quiet and precise.
I really didn't think that this upgrade would make that much of a difference, I just wanted the shorter cable routing for real life rides and races. But I have to say the upgrade to the R9100 is all that.