There's a lot of discussion on this for XBMC if you are ok with using that instead of Boxee (I personally prefer PLEX/XBMC to Boxee's UI)
You can buy the R1600 and install XBMC Live which is a LINUX distro and is able to take advantage of the GPU encoding. People report success with that regularly.
Just keep in mind the caveat to the R1600 for me is the like of wifi and remote sensor, so unless you have a wired connection available you will have to solve both of these issues with usb dongles which can ugly up that machine
Let us know what you do! (I'm considering trying the R1600/XBMC live route for a streamer in my bedroom to avoid the cost of another MacMini)