Private Pilot:
If you read one of my earlier replies you will see that I acknowledged that I have no studies, as I believe none exist solely for Motorcycle exhaust pipe sound levels, with a coorelation to increased rider safety. I did say that I would suspect the manufacturers engineers have likely studied it. If it was to provide a good "safety" feature then surely they would consider adding it even if only as an option.
I get the tractors have increased blind spots. But over the decades they also have improved sound dampening. I can only imagine how loud a pipe must have to be as the other poster, (who also drove tractors) said to defeat the nosie of your engine, CB, music, and any other devices which may be operating in your vehicle.
But again I would porefer a new rider such as the OP take the time to develop solid riding skills and abilities rather than rely upon loud pipes for his safety. Just as I would hope a new driver would actually learn how to park a vehicle rather than rely solely upon "park assist" to park the vehicle for them.
A rider with good skills and abilites is going to recognize the increased risk of riding in a tractors, (especially one with a trailer), blind spot. It is better to "hang back" a bit and wait until you can pass the unit in one quick and smooth movement rather than rely upon sitting there with your loud pipe and "hope" the driver has no other noises or distractions which may prevent the driver from hearing them.
But again to be clear, I am not opposed to a louder pipe on someone else's bike, (It won't affect me in the least). IF that pipe is "reasonable." Most of the time I have found that those who think a loud pipe will save their life are those who run straight pipes or have removed all baffles. Which would make sense after all if a "loud" pipe is going to be relied upon to ensure everyone hears you then doesn't logic dictate that it be as loud as possible?
A good example of this, is I live on a busy street in Oshawa. About 10 pm a month ago I heard a sport bike go by my place. My first thought was... "Man that is loud" I paid close attention, I could hear his exhaust, and from my knowledge of the area, I estimated he was at LEAST 2 km down the road before the sound returned to what most would consider a "normal" level. That pipe isn't going to save his life, BUT it surely will piss off virtually everyone within a 1 km radius of him/her. But I am sure the rider would claim they have that pipe to save his life... Of course MOST riders are going to be responsible with their pipe and ride accordingly, but it only takes a few and the regs will come.