With your "symptoms"
"Usually happens when the engine is warm
Gets a bit louder/higher-pitched on higher RPMs
(NOT!) Cannot reproduce if the car is parked and I rev
Sounds something like a tea kettle noise
Stock exhaust pipes, stock intake system except for K&N drop-in filter
I triple-checked the filter to make sure it sits properly and that the box closes fully
I also plugged the pipe that leads from the intake to the cabin, and it didn't help
No matching symptoms with an intake/exhaust leak
First thing I'd do is get your hands on a back pressure gauge that you can put in your O2 sensor before the cat. Do it cold then let it idle and get to operating temp. Check where gauge is, roughly 4-6 psi is acceptable. If it's higher or pegging the needle and fluctuating I would replace your catalytic converter. Too much back pressure can leak out of gaskets causing your symptoms. Just personal experiences. I work in a shop not saying I'm 100% sure but I've seen it often on other vehicles. Havnt seen a case on a frs or brz but it could happen. And in my frs I've put 22k on it back roads and a few canyon runs but never had 23-26 mpg I'm sitting around 29-31 with k&n also so it could be I drive like a grandma but I'm leaning personally towards a cat.