well normally a 780ti is definitely stronger than the 290x, but with mantle bf4, and when titanfall gets its mantle patch, the 290x is currently faster with mantle.
that said, 290x's dont seem to offer very much headroom for overclocking with air coolers. the 780ti classified will overclock like crazy, above 1.3ghz on stock air. if you waterblock the classy, the sky is the limit, likely 1.45ghz stable and cool, 1.5ghz+ if you like pushing it. with this type of overclocking headroom, the overclock will make up for most of if not all the performance increase that mantle offers the amd route.
the 780ti@1.3ghz is roughly equal to sli 770s in synthetic gaming benchmarks that offer >92% sli performance scaling. gaming obviously is different and many games only offer 70% performance scaling.