^ yes indeed. The Di2 RD-6770 (short cage only) needs a longer cage to handle the extra chain slack.

I tried ordering some RD-6700A GS (mid-long cage) parts for $30 from Shimano, but they are out of stock until some future date - maybe January they said, so I went ahead and bought a full 6700A GS instead from a shop in CO for $95. I received a 6700A but SS version, which I need to return - but did go ahead and test the cage off that SS model to ensure it does fit the RD-6770. The 6770 cage seems to have a lower top jockey wheel and the cage stop in a slightly different position than the 6700 models, but the 6700 installs and rotates without any issue. I can't figure out why they made those 2 changes on the 6770, because those would only seem to effect the cage when no chain tension is there to pull the cage to its normal "sprung" operating position. Since the 6700A is designated with "30T" max sprocket size (and people report success using 32T cassettes), the fact that I already confirmed the cage fits indicates this modification should work fine.

I may still install a 10T top jockey wheel just to improve clearance for if/when we ever use a 34T cassette. All told, this mod comes to $95 + maybe $35 for trick ceramic bearing KCNC jockey wheels, and this should work for up to a 34T cassette.

Going this route is a lot cheaper than the $335 that KCNC wants for their RD mod, and that mod apparently voids the Shimano warranty. Swapping Shimano cages is a piece of cake, and maybe explains why they are currently out of stock on the parts.