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I would tend to agree with Daox that it might be your #1 O2 sensor, which is the one that provides feedback to the ECU for fuel trim. They tend to get lazy with age but will not generally trip a CEL until they are really bad. The second sensor is part of OBD2 and checks the exhaust post CAT.
The popping in the exhaust on deceleration would indicate there is unburned fuel, which is especially significant since it should be in DFCO with your foot off the gas pedal. You can rev it up and listen to the injectors with a auto stethoscope to confirm DFCO.
While premium may be helpful, it should not be necessary even if it is recommended. The wife drove Muranos for 60k with regular when premium was recommended without issues and good mileage, occasional spark knock when dead cold and backing out of our fairly steep driveway.
regards
Mech
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