Let me try this again…
Weather DOES NOT matter as long as the tuner takes into consideration AID DENSITY and AMOUNT OF FUEL being used. In the OP’s case the tuner would also have to properly dial in BOOST w/ the MBC as the SUBARU ECU can NOT adjust boost once the OEM BCS is removed. IF you keep the OEM BCS (use GrimmSpeed/Perrin/GM 3 port solenoid for better control) then the ECU can also adjust boost depending on outside conditions. The ECU can/will switch maps depending on octane of fuel (knock sensors can detect this, switch ECU from ‘Table A’ to ‘Table B’ to compensate) for several factors such as timing/boost/fuel/limits/etc… Any known/reputable tuner will tell you this and will tune the car in accordance to the weather. Cars will always run more lean in winter (more air density) and more rich in summer (less air density) so the tune needs to be setup inbetween the 2 in a comfortable zone. This AFR will be set in accordance to each vehicle on a case by case basis (despite popular belief, there is no exact number for all Subaru’s…each car will have a zone it prefers to run in depending on motor/mods/how car has been treated/maintained). The tune is only as reliable as the driver behind the wheel who is responsible for setting the MBC to control what the car was tuned for at time of visit.
I hope this helps.
Cheers,
Justin