Depending on the muffs there isnt that much of
a HP drop.. just a few HP... I did testing at work
checking the difference.. but on those engines it was 6 hp
difference... in the end your muffs can and will make a
bigger difference.... there was a article on a Pontiac that
showed what little difference there was in ET just in changing
the tail pipes... then they went back and did the same tests
with different muffs... and with certain muffs and the 2 1/2
pipes... the same muffs and the 3" gained less than .02....
I want to say they were Dynomax muffs... but part of the
test was noise also.... I cant remember all the data on it
but it was a good article... it was all at the track
EDIT
and your right the temp is less so the volume/area
is less... the longer the car the cooler it is