It seems very difficult to trace the the 1/4in. circumference limit back to an official Subaru statement. The best I’ve been able to find is a forum reference to a Subaru internal document intended for service personnel, but not the document I.D. itself and several dealerships I’ve asked don’t know of the doc. Tire Rack, Discount Tire, and others list the circumference limits as 0.25in for Subarus as opposed to 0.5in. for most other makes - a potential explanation is that the Subaru system is engaged full time and thus always “slipping” whereas others wait for detection of slippage at the wheels before engaging (but this is beyond my direct knowledge). I know of one person, who races cars and has a complete shop, whose personal ride is an older Outback with manual transmission (I believe the MT versions have a different coupler design than the AT’s) that had mismatched tire brands when he purchased it used. The coupler failed, attributed to the mismatch, and he replaced it himself but the module was pricey.