>>1256663i like your extremely practical bike, and as such will bestow upon you some knowledge
10-speed sram mtb rear derailleurs and 11-speed sram road read shifters have the same pull ratio: i use an x5 rear derailleur with force 11 at the front on one of my bikes and can take an 11-36 cassette with that setup. i'll be matching an xo rear with red 11 on my next. also, the cable routing is much, much cleaner on the mtb kit, similar to shimano shadow geometry.
10-speed xo rear derailleurs regularly go for very little on ebay because no-one wants 10-speed kit any more. there's a 20g weight difference between them and the red 11 model, and you save that by default as you use less cable.
i don't know why more people aren't running a similar setup - i tried it first without knowing for sure it would work after looking at the numbers in this article:
http://blog.artscyclery.com/science-behind-the-magic/science-behind-the-magic-drivetrain-compatibility/ 