That is good news from Canon. I experienced a similar issue with my 1DX and an EX580II flash. During my last outdoors senior portraits photo shoot, I noticed that my flash was struggling to give enough light for my shots no matter how much I cranked up the FEC or how close I got to the subject. I had to do quite a bit of correction in post production.
A few days later I did a test with my 1D4 and my 1DX and I noticed that with the EX580II flash externally shot inside a softbox on a stick with radio poppers as triggers (always use them), the 580/1D4 combo exposed the scene perfectly and the 580/1DX was waaaaay inconsistent with many under exposed shots. Switching back and forth produced the same results (1D4 was perfect and 1DX was way off).
I now need to figure out what settings or combos are causing this but for now I am shooting all off camera flash with the 1D4. Glad I didn't buy that second 1DX and kept the 1D4.