That's why One-up has now announced a 45t rear cog for Shimano's new 1 x11 XT group. In the same way as the previous 42t extender, it goes inside the 11sp cassette and then you replace the 17t and 19t with a new 18t cog. You can do this with the new 11sp (11-40) XT cassette because it is not pinned or milled from a single block of material like their SRAM counterparts. The gears ramp more smoothly than the 11-42 10sp extended cassettes because the tooth gaps are more consistent between cogs. you still retain 11, 13,15 as the 3 smallest cogs so there is less of a cadence jump between the faster gears than the 10sp converted cassettes which went 11,13,16,19.... This all gets you within 3% of an XX1 cassette's range at a lower price without having to worry about XD freehub/driver issues. The idea behind One-up's conversion is that, compared to XX1, the 45t granny allows you to run a bigger front chainring which in turn gives more speed in your smaller cogs, somewhat negating SRAM's 10t advantage.
