It's called having a "straight pipe". From what I can gather it isn't that much louder. You really on;y get a big increases in volume if you cut the stock headers or install a full exhaust. Slip-ons don't add much volume, they mostly just change the tone of the exhaust.
Here is a thread on it for our bikes: http://www.r3-forums.com/forum/546-exhaust-fuel-delivery/3890-no-exhaust-muffler.html
Here is a thread on it for our bikes: http://www.r3-forums.com/forum/546-exhaust-fuel-delivery/3890-no-exhaust-muffler.html