Okay, I think you just talked me out of it. I do believe that it might marginally reduces the noise level, like you said 5 perc. There is no way that it could reduce from 80 to 50 db like they claim.
I tried putting the butyl on the rear wheel wells (I don’t have the booming problem caused by the trunk) like you recommended, but so far my measurements showed no discernible improvement. Maybe, my way of measuring (using my iphone) is not sufficient to detect the reduction.
Like I have mentioned in one of my previous post, I do have some experience as I had to soundproof my house heatpump, and I was able to reduce the noise (from high 60s to high 40s db) after several layers of butyl and an OEM blanket, but wrapping around the compressor is easy.