Don't know how is measured but C&D's lightning lap tests cars for sound at 50 feet all the same way. The GT350R was the loudest ever measured at 104 db. Porsche GT4s, GT3s and Ferraris are pretty loud as well. At MRLS, the most problematic stock cars that I see get flagged are the GT350/R, GT3, and GT4.
https://www.caranddriver.com/feature...p-2016-feature
https://www.caranddriver.com/feature...l-data-feature