Firstly , changing from hd 7870 to 270 x will be a downgrade as 270x is just another hd 7850 . Both the cards you listed perform much less than your gpu.
Secondly, what sort of help are you thinking the directx 11.2 wil give to you . Directx 9 was revolution in graphics while i dont see so much difference in direct x 11 and 9. Only some textures have been improved.
So, you think that directx 11.2 would revolutionize your gaming experience