I don't have an AMD chip that supports avx. My referring to AMD FX-8350 was just me lining up prospective cpus. Wishing there were a dual socket AM3+, to have processing power without so much cost. But I'm leaning toward building an i5 4690k, like you suggested. I'll keep looking as time goes on.
AVX2 isn't really what helps with SOB. AVX2 processors have FM3 which is what is helping speed up the processing over AVX. I'm not sure which AMD processors(Not my cup of tea) but many of the newer ones have FM4 which should help but to what degree I don't know off the top of my head. The AMD processors are cheap on the purchase side but it will end up costing more in the long run due to power costs. Even cheap power can't make up that difference.
The problem with many dual socket MB is memory bandwidth. It is better to have two desktops vs a single dual socket system.