So, after doing the science (finding the right adapters and cables etc.) and tearing apart the Mac Mini with a pizza cutter (yes, you read right!), I finally have my fast drive (Wester Digital Raptor WD1500ADFD 150 GB @ 10k rpm) and the Mac Mini is still as energy efficient as previously – mostly due to the fact that another FireWire drive is now in standby mode most of the time.

That should make me the first person with a WD Raptor in use on a Mac Mini. W00t! ;)

Obligatory XBench results that mean absolutely nothing:
Madness
Comparison with modded 7200 rpm Mac Mini

I’m using the Pleiades Super S-Combo external enclosure with the WD Raptor. The enclosure is made of aluminium. No cheap plastic parts here. It’s pretty solid and the aluminium functions as passive heatsink. That way, even under load the Raptor won’t cross the upper 55°C specification limit. My initial thought was that given the S-Combos universal FW400/FW800/USB2.0/eSATA-bridge it may perform worse than when connecting the cable directly to the drive. This is not the case. Actually, some benchmarks performed slightly better. I can only assume this being due to some sort of caching going on inside the bridge.
Also, if you think the drive is loud with its 10000 rpms, I can assure you it isn’t. Actually, in idle mode it is very much on par with the latest Seagate Barracuda drives which have a reputation for being very silent. The only thing that’s loud are the access noises, but in my opinion it’s not annoying.