just wanted to update, that I'm able to get a accuracy of 2ms between each hop with xbee zigbee series 2 modules. Synchronization between end-points and Routers is in order of 0.5ms. I did this all in the application layer.
All I need is 2 Routers and 10 end devices. This gives me an accuracy of 2*2ms + 0.5ms = 4.5ms from coordinator to last end device. For my application enough. For better results you need to have a deep insight to the mac-level I guess.