I can't say that I'm a fan of Mini-ITX for gaming. Micro-ATX is both more expandable and less expensive.
For lightly-threaded applications, the turbo boost of the Ivy Bridge processor will match your existing overclock at 3.6 GHz. With better IPC performance, that gives Ivy Bridge the edge there. With all four cores working at 3.2 GHz vs. 3.6 GHz for the overclocked Lynnfield, we'd probably have to find some benchmark testing on the web.
I'm going to say that the 2x8 GiB of memory vs. 2x2 GiB will help with some games, too.
i7-6700K, NT06-Pro, GA-Z170N-Gaming5, 32 GiB, 980Ti, SM951, 5TB HDD, Blu-ray, FTZ01, SX600-G, 3007WFP+2001FP, RK-9000BR, MX518