I think it depends on what muffler is used and the character of the bike. I put on an Emgo reverse cone. $65 muffler with no cat. The bike behaves perfectly, no dead spots in the power curve, about the same noise level as stock, same top end, BUT fewer vibes. Others have put on other mufflers and had poor results. Someone else may have used the same muffler and it was fine.
Without a cat you will definitely get more emissions. You may get a little more power from less back pressyre but it seems the major restriction is the header pipe. Without changing that you're still largely choking the motor. If you do change that YMMV. Depending on how your bike is set up and how it was built, it may work well it may not.
All that said, the D&D full system which is a low restriction muffler and header pipe combination seems to have given good results to those few that have installed it and reported it. I would guess it was also the only one developed on a dyno by a manufacturer just for this bike.
Scott