[Keito]

Have you thought about changing the transmission fluid, to freshen it up and possible switch to a synthetic?
Not sure what's in it from the factory but at 116,xxx miles it's probably beat.
My car has an auto trans and I just replaced my 40's series tires with 45's and
my mileage decreased by 1-2 mpg's driving 80% highway.
Might have a different affect on a manual but at cruising speed my boost gauge is now showing around 7-8 in of vacuum and with the 40 series was around 10, so considerably more load on the engine. I appreciate the increased ride quality though.

You could always unhook the wastegate actuator arm so the wastegate is flapping open and drive it around N/A for a while and see what the mileage is.