Your idle won't be as smooth, and decellerating will feel realllllly weird. I run a racing cat, sometimes, and notice that to be a major problem. Like was said though, you don't gain anything from removing it (I run the racing cat on the track because it's the only thing non-custom that fits my 2.5" collector) and if you do run it, it's really good for the environment. Your engine will also run better with it in, as your ecm will run closed loop, instead of dumping fuel because it thinks your O2 sensor is shot.