I would think they would stay the same or decrease ever so slightly (I've read somewhere that SATA 3 had some minor improvements relevant to this beyond just increasing bandwidth).
If you want me to, I could post some benchmarks of my Samsung 840 Pro (256GB) and 850 Evo (500GB) that I run off SATA 2 ports in my Core2Quad rig. The 850 is pretty much chock full, though, and the 840 is only slightly better off. I keep too many games installed waiting for benchmarks after I get a new CPU
