We are glad this question has been asked, because we love to talk about the nature of latex and how it works for slingshot shooting. The starting point for this answer is that actually thinner latex generally retracts faster than thicker latex, so thicker latex is not a solution for speeding up your shot.
So the first part of our answer is that thicker latex does not necessarily give you more speed on your shot.
The second part of the answer is that latex performs best in the higher temperatures. You won't need to do anything to get more performance out of latex in warmer temperatures, because it always works better in warmer temperatures. The reason we are all interested in cold weather performance of latex is that cold weather slows latex down. If you are in a place that is warmer, good for you. He won't have any problems with your weather slowing down your shot.