Hey there. I have exactly the same system, with four SSD's. My graphic delay is between 7-13seconds per click Since Yosemite. Its so awful that Ive stopped making as much music as I used to. I swapped out a GT780 at the same time as upgrading to Yosemite (for Kontakt) So I cant give a definite about weather a new vid card will help. Genius bar are also useless as they dont support Logic Pro or old machines such as ours. However:
The forums have been full of this problem across all systems, including new ones, for about two years and no solution has been reached. A few career Logic friends have swapped or are re-rigging some major studios over to PT and DP here in london as apple have been absent from its pro user base for a while now.
I am considering following them over to Performer but before leaving apple, PLEASE let me know how you got on with upgrading your VideoCard! I dont wanna leave just yet!!