As soon as you add anything to the game server it hurts performance, there is no getting around it. This applies as soon as you install a framework such as UMod, and ANY plugin. Therefore the question should be, how much does GamingAPI hurt performance? You can take a look at the benchmark here. The architecture of the plugin is to use multi-threading to perform the event and error handling, to be as performant as possible.

Yes, you can easily use other plugins along side GamingAPI as GamingAPI does not affect other plugins in any way.