So here is what I have been able to figure out. Based on this post over at Troikatronix by Mark Coniglio (developer of Isadora): http://troikatronix.com/troikatronixforum/discussion/1484/handy-adapter-allows-normal-performance-on-a-headless-mac/p1
According to this post, the OpenGL drivers do not load on a Mac Mini if it does not detect a display attached. The link above details a hack that simply plugs a resistor into 2 holes of a mini display port to VGA adapter (not kidding), and allows them to successfully run headless at normal speeds (users were experiencing poor performance with Isadora evidently).
I tried that and it did NOT work in my situtaion. However, what DID work, was this: I happened to have a DVI to Composite video adapter laying around, and also a Mini Display Port to DVI adapter. I plugged them in together, and hung it off the Mini, anbd voila! it all works as expected.
This kinda does not make sense, and I was wondering if anyone can explain it. I know the jit.gl.syphon object must rely on OpenGL… so I guess it is the same deal as the Isadora issue outlined above… Open GL drivers, if not loaded, will break the Max Syphon object. Somehow the Composite adapter is fooling the Mini into thinking there is a dsiplay attached. I am attaching 2 pics of the set-up. One is the “double-dongle” and one is the LED array I am running via the Max/Processing/Serial-USB/Teensy connection.