Pivos XIOS DS
runs XBMC however it is still in it's infancy but slowly improving.
The UI runs at 30fps compared to 60fps on a PC running XBMC stock skin, it's a little slower as a result but still faster than the likes of a WDTV. Whether XBMC rendering performance will improve over time with further optimization I couldn't say (it runs the same on the Android and XBMC-Linux only firmware), but you can use other lighter skins on the XIOS DS, for instance the Quartz skin which looks like the AppleTV UI runs at 60fps on XBMC only firmware.
The other issue is the media library, with such a large library your likely to use up the internal memory of the XIOS if it tries to store all the fanart/coverart. There are three ways around that;
1) Use the save media library to sdcard modification
2) Have an XBMC compatible media manager scan the media on the NAS and save the artwork locally to the NAS.
3) Deploy XBMC UPnP server
- it's hoped in the long run this will do something akin to Plex media server but at the moment all it gives you is a copy of the media library through UPnP as scanned by the XBMC server so you can only see it through files mode, the movie/tv libraries of XBMC wont be accessible. It's not a bad first step but not exactly ideal yet.
There are faster Android devices in the works from other vendors but none of them have XBMC developers actively working on them like Pivos as such they are a big gamble as there are lots of unforeseen issues. You could try asking Pivos aasoror
if there are any plans for an 8726-M6 box from Pivos, this is the dual core CPU/GPU variant of the chip used in the XIOS so in theory it should be faster, also a lot of the OEM 8726-M6 boxes
can take an internal HDD too.
I almost forgot there is also that Ouya Android set top box/console, that has a Tegra 3 chip in it and XBMC will be available for it but it remains to be seen how well it performs.