straight pipes are, when at full throttle, can reach 140 dB. My friends ninja 600 makes about 130-140 dB with his 4" straight pipe. People can hear him a mile away and it sounds not that bad. He has a full muzzy system and says he can notice a significant power loss with the straight pipe on.
Also, back pressure is important to the valves as it provides pressure for the valves to close on time. If you don't have then engine mapped correctly for the difference of exhaust flow, you could be seeping in exhaust gases back into the engine chamber and cause a reduction of power. But this is my understanding as I'm not an engineer.