sandy overclocks higher, ivy is 5-10% faster clock for clock, i've got mine easily to 4.3 maxing at 60-65C, so in sandy terms 4.5-4.8, which I would suggest is on the edge of what is regularly achievable.
Ivy memory OC's better too.
with ivy you'll pci-e 3 as well, which may be useful for your next card beyond the nvidia 6XX and amd 7XXX's .
Pci-e 3.0 wasn't even on my mind, but good call. The whole point of this upgrade is to get the most out of my 7970 and to future proof. Derp on my part.