OK, let's be very clear...
Jeeper's Jamboree is July 27th thru 30th (450 rigs, trail is closed)
Jeep Jamboree is August 4th tru 6th (250 rigs, trail is closed)
Jeep Jamboree USA's Rubicon Trip is August 11th thru 13th (135 rigs, trail is not closed)
The first two close the trail to outsiders (see this thread
http://www.pirate4x4.com/forum/showthread.php?t=490562).
The last one is a commercial trip put on by Mark Smith's commercial tour company, Jeep Jamboree USA, and does not close the trail. Typically they will NOT be on the trail Saturday, the 12th, as they will run everyone to Rubicon Springs on Friday, the 10th, and everyone out through Cadillac to Tahoe early in the morning on Sunday, the 13th. They tend to keep the group together, and yes, it is slow going if you get behind them.
Either way, as I said before, I wouldn't intentionally choose to go the "backwards" way out Loon on any summer Saturday.