When it is hot out, the air is not very dense, which means your car in the endeffect have less power. This is especially noticeable in places like Arizona, where temps over 100 are very common!

I dont know what the optimum temp. is for tuning, but I guess it would be around 73°F. If you want a real professional explanation, ask Rob from Needswings or the Guys from Eurocharged. I bet they can help.