My PowerMac G5 with the FX5200 card has the same problem. I remember reading somewhere that the FX5200 is one of the slowest/bottom line cards that can do Core Image. Maybe it's too slow for Apple's liking so they disabled that feature.
Definitely a GPU and not CPU related bottleneck as others have said. I wouldn't worry about it I'm using a Mac Pro with HD5870 and have translucent menu bar disabled.