I get what you're all saying about your class all running together, but there are too many benefits to running the split format. (not the least of which is that most people really like it)
We had 106 drivers at this event and did 5 runs and a relaxed lunch. It takes every bit of four 1-hour run groups to do that, plus a minimum of 1 hour for a course walk, 0.5 hours for a driver's meeting, 0.3 hours between groups 1 & 2, 0.75 hours for lunch, and 0.3 hours between groups 3 & 4. That adds up to an absolute minimum of 7 hours not including setup and teardown time and not allowing for issues that sometimes happen. (timing problems, pushing busted cars off the course, etc) To accomplish that with a single session would require EVERY driver to be there all day from 8:00am until 4:00pm or later.
As for the afternoon being faster, I think it's mostly just that a lot of the really fast guys choose to run in the afternoon. Many of them drive in from out of town, or have friends who do and they want to run with them. (that's most of the BRZ/FR-S guys) Some just aren't morning people or have co-driver's/friends who aren't morning people and they want to run with them. (Ben usually wants to run Late, so I've been running late with him)
Of course, any event is likely to see faster times throughout the day regardless of course conditions (short of rain) simply because most of us tend to drive to a target time. If we know that 6 drivers hit a certain time, we know it's possible, and we'll chase that time. If we're sitting on FTD, or at least what looks like a respectable time, if we don't feel we've made mistakes, we might keep doing the same thing through all of our runs without pushing the envelope or exploring different lines looking for another tenth or two.
I think if we're driving in the morning, we all need to assume that someone in the afternoon is ALREADY faster than us, and keep chasing the tenths no matter how far ahead we might think we are. It's easy to get complacent.
But, if y'all really think the afternoon has an advantage... pretend it's an all-day event and you HAVE to run in the afternoon... and register Late session. Problem solved.
Personally, I think I do better running in the morning, at least in the summer. By the time the first Late run group starts at 2pm, I've been standing in the sun for at least 7-8 hours already! I'm a lot fresher in the morning. (that's on my list of excuses for "sucking more" at the past few events)
He speaks the truth.
That is knowing what time you need to beat. I think they call it "benchmarking" or some other term. 
LOL
