The problem isn't the video of the Blu-ray disc, it's the audio. Since the 9400m does not have audio over displayport (it's not disabled, it simply isn't there) the Mini cannot passthrough HD audio (the Mini's optical port can only passthrough compressed 5.1) and without a hardware audio decoder (which all set-top Blu-ray Disc players have for their analog audio ports) the Mac software has to extract the AC3 or DTS cores on the fly while also decoding the compressed video. This is an extremely cpu-intensive task and is what both Plex and XBMC do. My quad-core hackintosh can do this just fine, but it puts it under quite a bit of load.
If you have or plan to buy a receiver capable of HD audio (True-HD or DTS-HD) then you're better off getting a dedicated disc player (which of course obviates the convenience of a file-based HTPC system). I'm sure the next Mini revision will have audio over displayport (since the Mini is a repackaged MacBook - which just got audio over displayport); however, until the software (e.g., Plex, XBMC) catches up and can passthrough True-HD and DTS-HD AND you have a receiver capable of decoding those, then we'll still be in the same boat - the cpu requirement for extracting AC3 and DTS on the fly. Perhaps an i-series cpu (dual core, hyperthreading) could do it, but I couldn't say without trying one out.
For me, I'm now using Make MKV to repackage the video with AC3 or DTS cores in an mkv container. I've listened to both HD audio and compressed audio (i.e., AC3 or DTS) and my ears can't tell the difference. But I'm sure some people can. Every Blu-ray disc that I have has been rewrapped into mkv, ts or m2ts containers without transcoded video and with AC3 or DTS and my 2 gHz Core 2 Duo Mini plays all of them with only a few dropped frames at the very beginning of playback using Plex. Plus, those repackaged files are several gigabytes smaller.
