Okay, so I had an opportunity to try out expansion points in a dark reception setting. Before I get into it, let me again say how flawlessly this camera performs outside, and in lower available light settings. The tracking with AI servo is way more accurate than the 1DMK3 and better than the 1DIV. And the low ISO noise I was getting with the 1DIV isn't present (I had to process noise in shots that were spot on exposure wise to begin with shot at ISO320 with the MKIV). Backlit situations outside..no problem for the 1DX.
However, I'm still running into issues on a low lit dance floor where people are moving. My 1DMKII's and IIN's would lock right onto that stuff historically. And my 1DMKIII, it runs circles around the 1DX in THIS department (emphasis on THIS if you didn't catch it).
Maybe this is something that can be addressed with firmware? Perhaps it's a short coming of the sensor? Again, it performs flawlessly in every other department! It's incredible. I just wish it would lock on as quick or even 1/2 as quick as the 1DMK3. There has to be someone else out there who has noticed a lag. I can't be the only one. And believe me, I'm a huge 1DX fanboy!!!
My work around for the next wedding. Use the 1DX all day long up until the dance floor opens up (in dark settings). Then switch to the 1DMK3 so shots are not missed. At this point I'm usually in the 500-640ISO range and the image quality is great (really just comes down to MP's and I'm cool with that, for now).
Thanks for listening,