I had purchased a PW Flex TT1, 2 flex TT5s and an AC3 a while back and could not get my Sigma EF-500 DG Super or the Nissin Di622 MkII to adjust their power. The Canon 430EX I owned would work beautifully (Of course, it was on Pocket Wizard’s compatibility list).
The Sigma and Nissin flashes fire, but they always fire at full power, no matter what setting I chose on the AC3 or the camera FEC.

I had installed version 6 .00 of the Flex firmware a while back when it came out (a month ago or so) but never re-tested the Sigma and Nissis flashes. The compatibility list on the Pocket Wizard website had not changed.

I was preparing for a photo shoot and was testing all my gear, and I accidentally put the Nissis flash on a Flex TT5 unit. To my amazement, the flash power was adjusted! I was amazed!
I then quickly tested the Sigma flash, and it too worked. The power was adjusted as per the setting on the AC3 contreller. All of a sudden I have 2 more flashes that I can use on the TT5 units!

Horray!

If you have a Flex TT5 unit, and own either the Nissin or Sigma flashes I mentioned above, please test them and let me know if they work for you.
Just to mention it again this change has been in effect only since the firware 6.00 has been released for the Flex TT5/TT1 units.

UPDATE:
The Nissin works great; rock solid especially in manual mode. No HSS though, but they have their time and place.

As for the Sigma, I have some reservations. The shots fired are hit and miss. Some are proper exposure, some are not. There’s the occasional firing of the flash by itself. And the biggest gripe is that the pocket wizards fail to communicate with each other way too often. The dreaded red blinking light. I have not found a pattern as to why/how this happens. Maybe to much interference from the flash?
It’s too bad though, since these flashes are quite powerful and inexpensive.