The 780Ti was the fastest card from Kepler IMO but it's no coincidence that they shaved off 3GB from the overpriced supercomputer cards
Yes the 780ti would have looked exceptionally attractive with 6gb and who would buy a 4gb 980 if you could pick up a 6gb 780ti? It's a shame the market is this way now. In the past each new generation would comfortably supersede the old so people would want to upgrade because the new stuff was so much faster. Now we have so-called new gen that's 10pc quicker than the old and has to rely on the built in obsolescence of the old gen to force the upgrade.