I doubt the option will ever be added. But I can only dream. The only reason to ever use the DX9 client now is for better AA.

The DX9 client is hardcore CPU bottlenecked too though. Even with a GTX 980 and a first gen Core i7 @4ghz. Can't maintain 60FPS at any resolution.

The DX9 HS benchmark shows the same issue too, the DX11 client. BAM no more CPU issues and it runs a heck of a lot better.

And the biggest point for my suggestion too would be that the UI pass is rendered after downfiltering the prior buffers. Otherwise the scale would be fixed, but then it has to be upscaled. Then downscaled again.