Firewall and exhaust outlet area to sound insulate makes sense.  Here is
something to consider.  While you can use ANR headsets to cancel out low
frequency sounds.  You cannot protect your body from same.  Sound pressure
levels  cause vibrations, movements resonance etc. Just goe to a rock
concert or stop behind a teenagers blaring car stereo at a stoplight and
feel the experience. The soundproofing, while it weighs does absorb,
dissapates some of  the sound pressure levels that would bombard your body.
The results are that your body gets fatigued from that continuous vibration.
How tired will you be after two hops of 3 or 3.5 hour trips in one day.
Will the soundproofing be helpful here? Losing possibly one elusive knot of
bragging air speed rights might be worth it.

You have a choice when building a plane, if you want comfort, have a
pleasant flight, be fresh when you arrive, soundproof your plane to the
point where it is useable for your purposes will increase those benefits.
If you are building a race plane or for other reasons, budget limitations
then eliminate the soundproofing.

I chose comfort and added about 12 lbs of soundproofing and this 320 is one
of the quietest around and it gives you a pleasurable flight experience
without getting physically beat up from the heavy sounds beating on my
chest. At age 60, comfort wins out.

I installed two types of proofing.  A high density linoleum type material
with 5 mil aluminum on firewall, nosewheel well and cockpit floor under the
pedals. a 1/2 inch of foam with 5 mill aluminum over the above plus along
the fuselage sides forward of the instrument panel.  The remaining fuselage
area has the 1/8th foam glued to the inside walls to which the leather and
"Is it Leather" material is glued to it.  This has apparantly been
sufficeint to dull the excitation properties of the fuselage sides by the
wind. and the heavier stuff does likewise for the lower frequency sounds of
the exhaust, engine etc.

Bob Smiley
N94RJ