Confirmed. The RD determines 10 vs. 11 speed. So both of these scenarios work:
1. Full 6770 (10 speed) modified to 11 speed: add 11-speed cassette (requires compatible hub), 11-speed rear derailleur & 11-speed chain.
2. Full 9070 (11 speed) modified to 10 speed: add 10-speed cassette (and spacer) and 10-speed rear derailleur.
Thanks Brian! I assume in my case since my hub would still be 10-speed that I wouldn't need to use any kind of spacer.
P.S I'm still not clear as to when you need to use the 1mm spacer with Shimano cassettes. I've heard that you should use it on any hub that is SRAM/Shimano compatible...but not on Shimano only hubs.