To follow up, I was able to sync my G15 with a PocketWizard up to 1/1000. At 1/1000 the frame was about 0.5 - 1.0 stops darker than at 1/800 or below (where each frame was consistant). Since I set the flash at 1/8 I do not think duration had anything to do with the reduction in brightness and more likely to do with the signal latency inherent with radio triggers.

This is in line with what the Strobist was finding:"The easy way is to use a Pocket Wizard, which works fine but will in practice limit you to a max of about 1/1000th of a second."