I've always held to the fact that when I ride (stock exhaust) I can hear most of what is gong on around me. Every sense you can use to increase your safety. A good loud horn is more effective than pipes in my opinion. Loud colors yes. In fact I ordered a banana yellow mesh jacket for my summer riding this week. I'm just feeling too exposed in my T shirt and leather vest. Yeah I look cool, LOL, but my skin is exposed to elements and road rash in the event, and I'm not as visible to cagers.
In keeping with the topic, I have heard, but not seen the stats, that accidents happen more often to bikes with loud pipes than not. I don't have any facts, it's something someone once quoted to me.