well @john I’ve just been notified and I updated to Build 401 and … it 's 1000 times better! 
Not quite perfect, but whenever (much much less) sound goes out of sync (when it happens it’s always in between tracks or when skipped/ffwd by user) it’s far easier to get it back in sync (just pause/play all zones)
It downed on me to try and set the Crossfade time and all the other settings to the same value on each Airplay device ungrouped and the same to the grouped zone and it seems to be working better (so far).
What now seems rock solid is the link with the AppleTV, which before was easily lost, and regained only by Disable/Enabling it. Now AppleTV works every time without need to do so.
I know Airplay is not the best way to stream music because the SQ limitations, but it’s still a convenient way for people like me with an already Extreme/Express wifi setup in their home - for casual/background listening, of course.
It’s very late over here, but tomorrow I’l try more extensive tests and enjoy the new features in the new build.
For the time being, Thank you!.