Just to confirm, you have a 2015 Roadster with a slip on muffler along with 20776 tune +5% fuel. It appears you installed the tune and ran it before you installed the RamAir filter with no problems. The tune shouldn't cause any mileage problems just by adding the RamAir unless you forgot to install/plug in the IAT sensor or damaged it or the extension cable is defective?? I'm grasping at straws here but the IAT sensor helps ensure proper engine drive ability.
Here is one of the conditions a bad IAT sensor can cause from a Google search I just did. I'm not sure if it will always trigger an engine fault code though.
"3. Decrease in fuel efficiency
Another symptom of a bad or defective air charge temperature sensor is a noticeable decrease in fuel efficiency. The air charge temperature sensor sends data to the computer, which allows it to make continuous adjustments to the engine’s air / fuel mixture for maximum fuel efficiency. Any problems with the sensor can result in incorrect calculations by the ECU, which may result in a decrease in fuel efficiency."
On another note, I have a 2012 Roadster with tune 20355 +5% from Hanso with TORs and it gives me very good fuel mileage with no decel popping. This tune is virtually identical to the one you have with the top 3 gears unrestricted like 20776. I also just installed my RamAir but haven't got a chance to try it. Will let you know how it goes as soon as it stops raining.