I live in an area where there are a few sports bike riders, myself included, and also, a few chopper/cruiser boys.
At 11pm midnight odd if I hear a supersports go by, generally I can hear it but it's going so fast its a 2 second very very slight annoyance that disturbs my sleep
At 11pm midnight odd if I hear a chopper/cruiser, for some BIZARRE reason they decide to sit at junctions and BROBOROBOBOBOBOROBOBOBOROBOBOBBOOBOB BROOMMMMM BROMMMMMMMM BOROBOBOBOBOROBOBOBOROBOR for like 5 bloody minutes, and then they finally leave, the issue is because they're slow, they don't get out of earshot until a good 20-30 seconds after leaving, by which point I am fully awake and thoroughly pissed off.
Also it isn't just the dB level, chopper pipes reverberate a lot, and have doppler effects more, so even when they're halfway to birmingham from your house (unless you live in birmingham), the sound travels so far and you can really hear them. Sports bikes, while also noticeable, aren't as crass and in your ear.
Loud pipes makes you more noticeable. Why would someone make a peer reviewed paper about this? How would you set out controls etc? It will never happen. Of course loud pipes make one more noticable. Loud sounds snap drivers back into alertedness if they were day dreaming. If loud noises didn't do this, then police and ambulance wouldn't have sirens.
However I do agree that after a certain dB level, the noise is so loud you can't hear sirens etc yourself.
Everytime a biker suddenly appears behind me I notice them more often than not through the sudden noise level, and check my mirror to see a biker freshly appeared. I would check my mirrors before turning anyway so would've seen the biker anyway when needed, but the point is I am always reminded earlier a bike is present due to its sound. Every time, it makes me more aware.