We have been using the old (white) and new (unibody) Mac-Mini Server version with standard MacOS installed (10.6.7) and an OCZ Vertex 2 SSD installed in place of the 2nd drive (server version)

Test results are pretty much the same as Todd...
One output, 2 layers of 1920x1080 25fps

Two outputs are not recommended - but we have run 2x 1024x768 with no problems and 2 layers of SD content

However - you must understand the limitations of Mac-Minis - One cannot expect the same performance as a MacPro system.

S