I'm only guessing that you've performed your Windows Experience tests with the ISRT running in "Enhanced" mode. You may want to try "Maximized" just to see if the results improve.
I'm trying to remember (without calling up the appropriate dialog) how my own ISRT configuration fared. But I do recall right away my conclusion about it. I have the boot-system-software drive cached with a 64GB SSD (SATA-III), and another SATA-III drive (for DVR and other files) connected without any caching or ISRT. So obviously, my disk score would suffer for the second HDD.
One more thing. Did you say you used a 320GB SSD for the caching? I hope you're not giving it double-duty -- as both cache and partitioned SSD. I believe this is a bad idea, although people have tried it and post a procedure for it. As far as I know, the Z77 motherboards still only allow a 64GB SSD cache, and I don't think it would be different for socket-2011/X79. Correct me if I am wrong on this last point.