well, as hard as I have tried, I think I am going to have to give up on this.
My HTPC just isn't good enough for live TV on my 4K TV. I finally got the PQ to be "close enough". The HTPC still has a more "over-processed" look, whereas the box just looks cleaner. But color-wise, and quality-wise, they are near identical.
What I cannot accept however, is that judder & image ghosting is too obvious on the HTPC. I have spent countless hours trying different combinations of settings, and it doesn't match the performance of my cable box.
I get best motion performance on my TV when I use auto motion plus, which is just samsung's fancy word for motion interpolation. With it off, the HTPC performs near identically to my cable box. However, with it on, the HTPC simply fails to deliver. Noticeably more judder, and ghosting appears. Even with very mild motion interpolation (judder reduction = 0, blur reduction = 10, which is recommended by cnet as best motion settings for my set), judder & ghosting is still worse on my HTPC than my cable box.
Best performance comes when I have xbmc set to match the TV to the framerate of the picture, and with vsync turned on. Those settings have the biggest positive impact on judder performance. Also, setting my screen size to 1080i also has a big impact, since if I try to watch TV at 2160p, the HTPC chokes. Outside of those changes, no combination of filters or upscalers have any sort of major impact on performance. Even with the most performance-conscious video card & filter settings, I still get judder & ghosting.
So frustrating. I don't have these problems with downloaded content, just TV. The judder only seems to happen when the picture changes between interlaced & progressive scan framerates.
Oh well, I will just use the cable box for my living room 4K TV, and the hdhomerun for my other rooms & devices. Might also use it for recorded content as well.
I hate to admit defeat, but unless someone has something that I haven't tried yet, I'm giving in & keeping the comcast box.