Some tests...

A test machine with one 970 and one 750ti, Ubuntu 14.04, ccminer 45, quark algo, compiled with both 50/52 capability
default config (without -d and -g parameters): no performance increase and validate errors
-d 0 -g 2 (only 970 runs): 35% performance increase, no errors
-d 1 -g 2 (only 750ti runs): no performance increase and validate errors