mkiv supra max rwkw on original turbos?

Hi to all I'm new on this site and was wondering how far could i push my stock sequential turbos with only modifying the internals. I want to keep it as original as possible. I own the jap spec model which came out with ceramic internal for turbo and 440 injectors. im tring to get at least 350rkw if possible.Could some please help on what i could do. Thanks.