[noggin]

Hmm. Not sure what I was expecting from 4K from the Chromebox, but it does actually support those output resolutions, so I had to try. XBMC menus feel a bit... I dunno, not laggy, but perhaps the term is "non-fluid". Tiny thumbnails look fantastic. However...

What you are describing as lag is almost certainly the result of the output frame rate being halved from 50/60p to 24/25/30p. The Chromebox (like almost all HTPCs at the moment) can only output 2160p at 24/25/30p over HDMI - where as at 1080p it can output 50/60p too. As a result any animation or mouse pointer movement will be rendered at half the frame rate, so appear far less fluid.

Annoyingly the HDMI 2.0 spec adds a 2160/50-60p video mode which is compatible with HDMI 1.4 bandwidths. They do this by reducing the chroma subsampling from 4:4:4 or 4:2:2 to 4:2:0 (which is how almost all consumer video is encoded in SD, HD and UHD anyway). This is how nVidia Kepler and some Sony UHD TVs were able to add 2160/50p and 2160/60p compatibility to gear which didn't support those resolution/frame-rate combos originally. I think nVidia are the only people currently offering driver support for this mode on kit which was originally HDMI 1.4 only. I don't know if the Intel and AMD GPUs can implement this - or whether they will bother. It is only really useful for playing 4:2:0 video - and not great for desktop and general PC use (as the chroma is effectively only 1920x1080 resolution overlaid over a 3840x2160 luminance signal) so coloured edges will be softer/blurrier.

You are right, it would be good if there were an optional, "Match resolution" as well as "Match Framerate" option. It could be quite complex to implement though - as so much video has non-standard resolutions (where black bars have been cropped for instance). However you could do a bit of an "educated guess" system - and let people check box at what resolution they wanted XBMC to stop scaling?

You could have different settings for "SD" (i.e. content with a horizontal resolution of 1024 or lower?), "720p" (i.e. content with horizontal resolution >1024 but <=1280), "1080" (i.e. content with a horizontal resolution >1280 but <=1920), "Above HD" (i.e. content with a horizontal resolution >1920)? Then you could have "Output at" options for these four genres (So you could chose to output "SD" as 1080p, "720p" as 720p, "1080" as 1080p but "Above HD" as 2160p?)

One thing I've noticed with an HTPC connected 1080p to a 4K set is that when playing 720/50p content you get a double scaling 720p->1080p->2160p. If you output from XBMC at 720p you just get a single scaling - 720p->2160p and the result is cleaner. (You can't output 2160p direct from the HTPC as you can't get 50p output over HDMI unless you have the nVidia solution - and 720/50p at 2160/25p looks nasty)