All the 512 bit bandwidth does on the 290x is allow the speed of the memory to run lower, thus allowing the same effective speed. The 290x runs at a 5Ghz effective speed, whereas the 780Ti runs at a 7Ghz effective speed. There is almost no scenario (besides cryptocurrency mining) that the 290x will beat a 780Ti, so it is WELL worth the extra $10. Mantle supposedly offers significantly increased performance, but that will only work on games that support mantle, and we have no realistic numbers on that whatsoever, therefore I cannot offer any advice which includes mantle as a factor.