Here to put this one to bed...
I owe a 7D and a 5D Mark 3. I shoot with a 580EX2 flash. I am the in-photographer for several nightclubs, including one of the O2 academy venues in the UK.
This problems exists. FULL STOP.
If I'm doing a portrait of an artist, DJ, or just patrons of a venue. I will shoot with my 5D3 (I prefer the camera) If it's spontaneity that I'm trying to capture, I revert back to the 7D. I have shot with my 16-35L2 24-70L2 on both bodies, same night, same venue, and found, time after time, the 7D finds focus quicker.
Day time, different story, as far as I can tell, this is an af beam assist issue, and I further invite any Canon rep contact me, and show me how this isn't a problem
Steve...how about just plain low light without flash between the two? Have you done that comparison? As I said, I've had problems with a flash attached, but low available light shooting has been very good for me and I can't believe the 7D would be better with flash off in low light?
First off, can I refer you to a post I made around 5 days after launch (I received my copy on launch day (Keep reading light leak issue not the problem here))http://www.canonrumors.com/forum/index.php?topic=4811.msg95784#msg95784
That's when I first noticed the problem... and it was bad enough that I thought that I must have a defect copy, and it went back very quickly indeed.
I had to wait awhile to get a replacement copy as Canon froze shipping, to address the light leak non issue (I sent mine back before I even knew of this, and for the AF reason only) in-between me posting mine back, and receiving a new copy.
Long story short, new copy still has issue... I lived with it, but it's definiately there, to the point, I couldn't do what I do (Professionally, mainly nightclub orientated work) with only the 5D3, soley due to this problem. The 7D still gets alot of lens time. It's a hassle at times, but what can you do?
Secondly, as far as I have experienced (I have shot a few weddings with my 5D3, along with a few other bits and bobs) and camera without flash attached is a dream. AT LEAST as good as the 7D speed wise, and finding focus without any sort of AF assist, where I would have to prey that the AF assist could help my 7D enough.
Remarkable piece of kit in every day use, and low light.... just not really really low light!