It won't do any serious damage to the engine. I've had it this way for about 5,000 miles now and have had no trouble. It could effect your gas mileage because of the o2 sensor not seeing the voltage it needs because the cats are not there to do their job.
And it did make it louder, but I really don't know how much because the 40 series is loud to begin with and my truck was down for about week and half to two weeks when I did it.