Kenton Grua and some other Grand Canyon Dories guides ran the Grand Canyon in 48 hours once. I think it was from Lee's Ferry to Diamond Creek.
In the high water of '83 I did a single-raft trip in 3.5 days (4 people on an 18' rowing rig). We weren't trying to go fast, just checking out the rapids for the Park Service (that's how we sold it to them anyway - I was an outfitter at the time). We ran all the rapids solo and camped out every night. We never even had to row downstream, just stayed in the current and cruised. We put in at Lee's around 5pm and floated to Nankoweap that night. We ran the Roaring 20's and President Harding in the pitch dark. It just so happened to be the night of the new moon. If I recall it was about 60,000 cfs and rising. President Harding was terrifying as we could hear the giant hole as we went by on river right.
A motor raft had flipped in Nankoweap Rapid that day in the normally small middle hole at the top. Needless to say we stayed just off the right bank all the way down that one, just so we knew where we were.
It was a fantastic trip and I'd love to do it again but I'm not counting on it.