I would try installing a second battery before installing a HO alternator. Search for xp2500, it's a 100 amp hour battery designed to supplement up to 2500 watt system. Also have you installed the Big 3 Upgrade?
I am also looking for NU9020 firmware update for a 2015 Legacy with Nav. Would anyone be able to email the files needed. Thank you in advance. ralexander1447@gmail.com