Yeah I run my 780 @ 1300mhz 24/7, if it's only a few extra cores and clock boost on the 780ti, def wouldn't be an upgrade for anyone on a 780. Especially as the 780's can clock higher than some Titan's meaning that this architecture @ 28nm is almost at point of diminishing returns, i.e more cores / less overclocking. My old Titan gave me similar performance when clocked as my 780, my 780 can clock higher so the extra cores of my Titan were negated.
Maybe a redesigned GK110 / GK180 with '2880' cores could bring more performance, but only if it's tweaked to give high clocks / better efficiency. I'm hoping that's what Nvidia deliver, not some in-between 780 - Titan waste of time card..