I have used a variety of Canon cameras (10D,1Dmk2,1Dsmk3,5D,5Dmk2,5Dmk3) with Buff products and various sync vendors(Buff,PocketWizard, Chinese triggers). None of the triggers gave me maximum sync speed with reliability. Outdoors I can fudge it but indoors I can almost always see a band.
In the studio I use 125th and outdoors I use 160th.
Just the nature of their shutter.
The Phottix and other triggers claim higher sync speed but it is not a high speed sync but rather a trigger timing adjustment that, depending on you camera and strobe combination, may allow you to sync at higher speeds. Ironically it seems that the best strobes for this are those with the longest durations thus negating to some degree the action stopping power of strobe.