One advantage of a solo trip for me would be spending much more time floating the river early morning and in the evening, times typically spent in camp.
Would also do many more short hikes. With a big group it is easy to stop at all the standard hikes/attractions, but easy to miss the less popular hikes and side canyons. After five trips I still have a long list of camps and hikes I would like to see. Also could stop at the crowed spots (Matcat, Havasu, Deer Creek, etc) very early or very late to avoid the crowds.
Although I would do more hiking, I would probably do less of the sketchy moves on some hikes. A sprained or broken ankle 5 miles from the river would be a major problem on a solo trip!
Could also camp at some of these stops by getting there late or leaving early. Always wanted to sleep in the harbor at Havasu. Would be no issue with finding camps, there are hundreds of spot where one boat, one cot will fit.
Would greatly reduce the amount of time spent setting up camp and preparing meals. Gear and food would be more backpack style than typical float and bloat.
My boat would be rigged differently, in that all safety gear would need to be accessed from the top and the bottom. Would travel much lighter than on a big group trip with a much cleaner load, just in case. Despite trying to go lighter, I wouldn't have someone else to cover if both flashlites broke or the security of having 5 plus boats with repair kits/ spare parts.
Did a Sept 09 trip with floats at a steady 10k for 24/7. Would have made a perfect time to run solo because you would not have to worry about moving boat in the middle of the night, or hauling boat back to river if you were a sound sleeper.
A couple nights at Phantom Ranch might be a pleasant break. Warm bed, shower and someone else doing the cooking! Please keep us updated on your plans and hopefully a trip report when you get back.