It would be very hard to generalize between a "bad" dx11 and the "best" dx 10. However, in this case, a gtx 295 would blow the 5570 out of the water. However, it does suck up much more power than the 5570.
Well lets see wow I have a bad DX 11 card's and would I take a very good DX 10 card? There is two answers to this puzzel. First My bad DX 11 cards could handle the DX 10 games better than the DX 11 games. Second answer. Not all games are totally DX 11 and the game that are DX 10 then yes I would take the DX 10 cards for the higher eveything.
These are all very good Answers, If i had the PSU and a good water cooling setup (Cause the GTX 295 gets hott) i would get the GTX 295 over the 5570 anyday. plus its still gonna be good till Maxwell or Keplers greatest inventions, then its gonna be like comparing a 8800GTX to a GTX 460, because what was kicking before becomes obsolete and whats kicking now becomes downgraded to kicking before tittle. and the ultimate GPU's like the GTX 295, the GTX 480, the GTX 590. All power hungry, hot tempered but yet last the longest..(In performance)