This is interesting.
I've started using JRiver with MadVR at 1080, and I'm seeing significantly less loss in sharpness when I use 2.35 aspect mode on the vw1000. I'm also seeing other improvements, like less banding and better audio-video sync, compared to XBMC.
One impressive aspect is that it does all video processing in floating point, then dithers down to 8 bits, so it does a great job with processing, including things like decoding, color correction, and gamma tweaks.
I don't have a 4k video card, yet, so I can't comment on how it scales compared to RC. Perhaps someone could post close-up pictures?