On my 350Z I recenctly designed and installed an expansion chamber to
the factory exhaust system.  By removing the section of pipe which
contains the resonator and replacing it with a 34" X 3" tapered chamber
to settle the exhaust prior to the muffler.
What an improvment,  a much nicer exhaust note,  slightly more
pronounced throat.  But with a significant increase in torque and
freedom to rev.
May be something to try for under $200.00  before you spend  $1400.00 on
a full system.