Hard to find a comparable benchmark, but here are some rough and ready numbers. And I really don't think Titans should be included in this discussion, they are not gaming cards.
This is again fake. GTX 480 and GTX 580 is technically one generation, split by the marketing. The difference is just one minor revision of the Fermi chip.
GTX 680 and GTX 780 Ti is also technically one generation. GTX 780 Ti is the real GTX 680 Ti.
Edit: See how the *50 buyers got screwed over the generations - at some point they experience performance degradation
Also, nvidia was fair with GTX 280 and GTX 285. The latter was a shrunk to 55nm chip but the generation remained the same.
After that, something bad happened to them (and to AMD, for that matter) and they began jumping over the generation numbers like mad and like no tomorrow. Meh.