Beat OCed SLI 670 by 20% not in this life time.
Let's play with some scenarios in dual card first, so these are the scores from our firestrike thread.....
11503. GFX Score 14326. 670. Air cooled. Physics Score 13270. CPU 2700K @ 5.2 Air cooled - DiaVolaAce - Link
15700. GFX Score 19332. 7970. Water cooled. Physics Score 17400. CPU 3930K @ 5.0 Water cooled - Besty - Link
Based on this information, you can speculate that with a 37.5% performance uplift over a 7970 a dual 290X setup under water with 1.35v-1.4v will produce an additional '7250' graphics score over my cards, so lets round that down to 7000.
7000+19332 = 26,500 'ish (rounding down again).
A score in that ball-park a heavily over-clocked 290X would be 83% faster than a the example 670 SLI setup with OC and trading blows with the titan.
Dial the overclock back on the 290x and you are sill way beyond a 20% uplift over the 670SLI, even accounting for differences in CPU speed on the example rigs.
Using a single card, and assuming the 290x is overclocked, you are looking at a 40-45% uplift over the 670SLI ?
Assuming the 290x is at stock well, its going to be interesting to find out, you are probably correct but its going to be by small percentages and the non-reference cards may get it to 21% at some point but I think it will be too hot on air to do so.