Yeah not entirely sure on that. It isn't even really the 3rd gen since it became a standalone model which kind of came about with the VA chassis, with the GR still having the same basic body as the Impreza which just moved to a new body way earlier than the WRX/STI.
Depending on how the sheetmetal is made behind it there is potential for quick swap moderate flares that actually match the body unlike 95% of current kits out there though.