We did the Wild Africa Trek at noon last July. It was hot, but no worse than any summer day at AK. In fact, with fanned areas and hydration stops, it was more pleasant than just walking around the park in the heat of the day.
Shoot for an early morning tour if available. We booked at the last minute and didn't have a choice of time for that day, but it was an I credible cperience and we would love to do it again. If the choice is between a July tour or not doing it at all, I'd do it in a heartbeat. For us it was well worth the cost.