personally, never understood the lure of having two complete systems in one case.
That being said. The 915 external is neat. At one time I pondered doing something similar with a Mountain Mods pedestal for a stand-a-lone ITX build, layout similar to Silverstone Fortress FT03B. Silly thing called a budget kept getting in the way
Are the cages in the external removable? Looks like they are screwed in. With them removed would make for loads of room for water cooling stuffs.