300+ amp Alternator for 2021 Crosstrek Sport 2.5L?

As title states, having a hard time finding anything over 250a, tried mechman and js alt and both tapped out on a custom one, is my only option really getting a universal alternator and getting a custom bracket made?

Going from current 500w RMS/2000W which is already capping my electrical out. I'm going up to 2000w RMS/4000w amp and a 400w 4 channel amp. Can I get away with the 250a alternator upgrade, or do I have to make the leap to the 300 custom? Not trying to skimp but also not trying to make it a pain in the ass.