DX11 in of itself does not add performance. Its especially not important on lower end cards that are using all of the cards hardware to play at 'normal' quality. The 5670 is a entry level card, with only 400sp, which directly compares to the 5770 which has 800.
The 5750 has 720sp. The 4770 has 640, but it has about the same rop's , tmu's as the 5770. Where has the 5670 has only half again what the 5770 has.
This chart is very informative.
The 4770 is compared against the 5770+5750 in the launch article of the 5770http://www.tomshardware.com/reviews/radeon-hd-5770,2446-8.html