What can be done to a direct injection engine without going into the computer? Anything? Have been told even an intake (tube and filter) need a retune for the PC.
I was thinking of intake, catback and maybe header. NA Japanese 4 cyl if it makes a difference.

My understanding is without tuning, you won't get the full benefit of less restrictive intake and exhaust. This holds true for carbureted and injected motors. With today's electronically controlled motors that means reflashing the computer. Used to be you had to tear apart the carb and mess with jets, needles, and springs.

Someone way more knowledgeable than me will be along shortly to give you good advice.

I'll bet there is a forum dedicated to your motor on the interwebs.