I didn't get to finish my post due to work issues....(pesky work stuff).
Anyway, I add dual exhaust on my 05 Grand Marquis. I called the Toyota dealer (bought it used with a dealer engine warranty) and they simply said the warranty would be void if I did any mod that would/could cause the issue. They would not be more specific regarding the dual exhaust.....
However "experts" on the Ford Forum for Panthers (Crown Vics, Town Cars, and Grand Marquis - all the same) strongly cautioned to stay with 2 or 2.25" pipes. Bigger was actually BAD for torque AND the valves.
.....least thats what they said. BTW I had the duals put on and it ran great for 60k like that .....now it is at the Dodge dealer waiting for a grandma who like duals with chrome tips.......