Check the condition of the secondary exhaust tip
I was just going through the car yesterday inspecting the rear suspension to make sure everything good and decided to inspect butterfly valve on second exhaust tip (vacuum actuated) and it was SEIZED SOLID!!! Not just a little seized, but completely siezed!!! No amount of WD40 and heat would free it, so I forced it open (chisel and sledge)...... gonna have to look @ but the reason for the post is that i've noticed a tremendous improvement in the performance of the car since opening valve. The ECU opens the secondary exhaust under acceleration and load. If it's jammed shut, the additional exhaust gases back-up and reduce performance and HP!!!.
Just the operation of the butterfly valves.... Should move with slight pressre on plate!!!
Car is breathing so much better and revs much more freely......