PowerPC CPU games should work fine. Gradius IV works fine on the PC with the interpreter CPU (although at less than 1 FPS on my P4-3.4 GHz) so it should also start up, albeit verrrrrrrrrrrrrrrrry slowly, on Macs. What does the LEDs do? It should start out saying "UP", then "Cx", then "11" for a long time, then "12", "04", "21", and finally graphics will begin appearing at "22", after which the LEDs will turn into a spinning beach ball.
ETA: tried it myself, the PowerPC is definitely running properly, but either the SHARC is busted or I'm not waiting long enough.