Well, the 3GB variant has the cut-back number of CUDA cores, TMUs, and SM units.
The 5GB version, oddly, a lesser memory bus and 8 fewer ROPs.
The 6GB and 6GB with 9Gbps RAM, well, yeah, memory speed.
The new one switching to GDDR5X *and* using GP104 instead of GP106 (albeit a cut down GP104).
Although, overall, I agree. I don't imagine differences would be huge (well, the lesser memory ones, maybe, in some games). Still, it seems these days that memory throughput is more of an issue than clock speeds.
Would be nice to see a comparison, though. I think 2560x1080 is going to become more common as ultrawide gets more adopted, and it would be interesting to see if these tweaks/upgrades make it capable enough for 1080 ultrawide with max details @ 60fps