Wow, that is a BIG drop. x2 on what griggs95 said. Change out the RDU fluid as soon as possible, hopefully it has little to no metal shavings in there. If pasty/dirty looking, R&R repeatedly with "cheapo" fluid like Walmart SuperTech until it stays clean, then final fill with fluid brand of choice.
Check the intercooler for crud buildup, clean the MAF sensors.
Spark plugs might be getting long in the tooth, I assume you have changed them once already by now? You can easily gain back 2-3 mpg if the plugs are worn enough.
Can you scope the valves to see if carbon buildup might be an issue here? There isn't really an effective dropin FI cleaner that will help. A GDI induction service would be my recommendation, it can also help gain back a good amount of mileage.
Has the coolant ever been serviced? Should be on 3rd set of coolant by now.
Any issues with the AC compressor? That can cause drag. Brake calipers and slide pins too.
All the usual things with any car: alignment, tire tread/pressure, removal of excess/unnecessary "cargo", ...
And of course pull codes, just in case.