I've been on one of the first safaris in the AM and one of the last before park closing. I have also been on one around 11am.
I, too, heard the same as you, that mornings were the best. And yes, we saw a lot of animals on the first safaris.
BUT...
Our best animal experience was on the last safari of the day (one of the last). We saw more elephants that trip AND the highlight was that the lions were actually far enough out of their lair on the rocks to get a good picture of them.
On the AM safaris, the lions were under the trees, in so much shade that the pictures just weren't very good.
On that last safari too, we had one of the rhinos pacing our truck. I am sure that probably can happen at other times of the day, too.
Everyone and their brother runs for the safari first thing...if we end up at AK inthe latter half of the day, I will always watch the parade from Harambe and then take the safari afterwards because that was an awesome experience.