They do have several 16GB Ripjaws X kits with different timings as well. I have 12GB of Ripjaws X in my P67 system (one 2x4GB kit and one 2x2GB kit), so multiple dual-channel kits do actually work together. Some people just get unlucky with a bad stick.
Until a few days ago I would have agreed about the Ripjaws X, but there have been too many problems with 2 kits of F3-12800CL9D-8GBXL edit: to be clear the problem is running them at 1600 MHz, 1333 MHz was okay, and has nothing to do with Memtest failing. It's weird, Google those kits.
I totally agree about any advantages of >8GB for games - none as long as you 'shared video memory' is maxed-out.