I have found HSS to be faultless in manual in pre 2012 bodies, but it does not work consistently in ETTL HSS.
Here is a test I did in manual HSS with a 1Ds MkIII, the model is cheap
It is actually three 600EX-RT's each in their own group and fired with the ST-E3-RT, just to see how it all worked and what I could expect from it. The most noticeable power "improvement" is the 200mm zoom, that is a great feature, the more I play with flash head zoom the more I like it and the more I wish the Canon system did it remotely, like the Phottix Odins do.
Very cool, and good point about the zoom function - though it doesn't matter as much after I drop them into one of my favorite light mods.