The exhaust flap is the only topic in this thread I can comment on, the rest is way over my head
That flap is on the right side of your dual exhaust and is a simple butterfly valve that severely restricts gas flow out that pipe. It's controlled by a simple motor bolted to the assembly and is super-easy to access under the rear bumper. It's held in place by 3 8 or 10mm bolts (I forget which size). The motor, not the flap. The flap is integrated into the exhaust. You probably knew this already.
The motor is connected to the flap using a rigid clock spring. that clock spring is the diameter of a 50-cent piece, made of coathanger wire, and is brittle as heck. It is very easy for that spring to snap, and then the motor can't control the flap anymore. Loosey goosey.
Under hard acceleration (including with a "lowly" 350 HP), the gas passing through that pipe is enough to make the flap move, as friction forces are not enough to hold it in place. I don't remember, if the flap gets pushed CLOSED under acceleration, or OPEN. If it gets pushed CLOSED, you could be choking off your exhaust, building heat down there, etc etc.
^^^^ this is how I originally found I had this problem. I was accelerating (normally) from a stop light in a 35 zone with all 4 windows down when the exhaust sound changed very very suddenly. the flap was snapping out of place (open or shut, I forget which). Could only hear it with the windows all down.
This is an easy hypothesis to test. In the driveway, pull the motor. If half a clock spring falls into your hand when dropping the motor, you may have a smoking gun. The loose bits of the spring were jamming up the motor, bad feedback on position, and thus the code thrown. The spring is not "user serviceable", the dealer makes you buy a whole new motor at $300. Folks on the board here found an infinity part# (I think) that's the same part for half the cost. You can search that out.
Though frankly your dealer should have already done this if they pulled that code....
If the spring is good, might be misalinged and locked your flapper closed. Then you just re-align at re-install and GTG.
First troubleshooting I would do is get the car nice and hot (15 minutes driving should do it), and pull over when in sport mode and check if the flap is open. It's super obvious when that flap is open vs closed based on hot gas ejection from the pipes. if closed when running sport mode and warmed up, you have a problem.
Everything in this post is only commentary on the hardware. I make no claims about how it relates to your power issue, that's above my pay grade. But that hardware, I know it. It failed on me 200 miles before my warranty expired and was the last and only repair done to my car on warranty.