With Thinkpad X250 you can select 16GB RAM option.
EDIT: aha, seems like a company called Intelligent Memory makes a single 16GB module [1] that works with X250.
[1]: http://forum.notebookreview.com/threads/now-16gb-ddr3-module...
You can select a 16Gb option for the X250. It does say "single SODIMM" though, which presumably means single channel memory => a very expensive SODIMM & sub-par performance compared to a dual channel equivalent.
> Memory: Up to 8GB (1 DIMM), DDR3L-1600 PC3-12800 1600 MHz