I have tried every word combination that I can think of and Google is doing a poor job or looking for what I am thinking about so I’m coming to the brain trust here. Does anyone know of or have a link to a test that shows what the decibel level difference is between a supersonic bullet and a subsonic bullet with all other factors being equal? Bonus points will be awarded if the test involves a short barreled, suppressed 300 Blackout.

For example most suppressor mfg. probably give you that data. I'm a surefire guy and for a surefire .300can (weapon unspecified but same barrel length) sub is 126 dba and super is 133 dba. And I suspect with a bit of work on the mfg. site you can find it for most cans.
But also over at PewScience it takes a bit of work to find both the sub and super for the same suppressor as they are in different reviews but the data is there as they only use a small selection of the guns for each caliber.
And lastly many reviews have it as well, but you have to suck it up and watch the review to find the data.