I've built SR-2 the minimum difference is $2,000+, it's great for a rendering PC but it's lack luster for Gaming and on par with 990X. My last SR-2 build was $12,000+.
I too run an X58 gaming rig with 980X and Koolance, unless the OP is running 5900+ x 1080 then the LGA 1366 is pointless. A full water rig is going to run $4,500+.
Problem, the above X58 MOBOs are limited to 3-WAY SLI therefore your only quad option is the GTX 590 SLI.
Q - What's the reasoning with the 990X?
Q - What's the purpose of this build?