I had my first chance to use Motion 3, FCP 6 last week. I have an 8 core machine with 16 gig of ram and the Quadro FX card. My storage is SAN using 4 gig Fibre Channel. About as much hardware as you can thow at this.
I found that templets from Motion don't render well in at all in FCP 6. Often the renders peg a single processor, and lock out the rest of the interface for 15 to 20 seconds at a time. Then the interface comes back, the pegged processor changes and the interface locks again. Render times for a simple 15 second animation are hours.
As for compressor. I tried using that to backgorund render Motioin projects. Unfortunatly about half of them had "glitch" frames.
For the moment I am sticking to rendering in Motion until the next upgrade cycle.
-Vance