There are 2 Ultegra Di2 rear derailleurs. 6870 SS and 6870 GS. Can’t find the difference between them. Anyone know. The GS weighs more and is more expensive than the SS but I can’ t figure out why choose one over the other.
The cages are different lengths. The SS will handle up to a 28T gear. The GS will handle up to a 32T gear.
I’ve used the SS with a 30, if that’s any help to you building off the other poster. With the right chain length it isn’t a problem.
This might be helpful to you if you are wondering what works with what. I just happened to randomly see this a couple weeks ago and it encompasses the entirety of my knowledge on the subject 
it sounds like the other poster is right, there is not an exact cut off despite what Shimano says.
Thanks.