You can gain a few HP throughout the entire RPM scale by removing your PS. However, long term effects on your steering system, which requires lubrication, arent ideal, and IMO, its a pain in the ass, especially if you ever have to parallel park.

As for AC, I dont have the belt on, and again, it isnt ideal for your AC system which needs freon circulated frequently (why it is recommended to run your AC in the winter). I havent had any problems nor did I notice any difference, but I also havent run the AC in 9 months :P