Heard some of the same stuff
been going to the same muffler place for years and have always liked the 50 series flowmaster which is what i just put on my 05 along w/ the K&N FIPK
AWESOME, love it
from what i what told the design of the 40 vs. 50 series and why the 40 is louder, don't know if there is a pic online but at this shop and maybe one nearest u if u look at one of them cut in half u can see that the 40 series is a what they call a two chamber and the 50 is a 3 chamber from what i have been told
heard the 3 chamber creates better performance gains and the sound is deeper
i would get only a 50 not a 40