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Something is wrong. With the degree of computer control in modern motors, it’s hard to guess. I would take it to the dealer and let them do some exhaust emission testing. This is just a guess, but if there is a leak somewhere that would cause a sensor to pick up an overly-lean reading, the ECM might adjust the fuel mixture richer, resulting in worse mpg. But for whatever reason, it seems the engine is running a richer mixture. If that is allowed to become extreme, it is potentially damaging to the motor, so you definitely need to get it diagnosed. Absent any other changes, a less restrictive exhaust system should deliver better efficiency. These motors are gas hogs at best, so I feel your pain.
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