could i use these on a brand new (not rebuilt) non-ported 13b-rew motor with the stock ecu and injectors with low boost?

 

Sure, you will just be limited to about 10-12 lbs of boost.

 

What are the AFRs going to be at 12 psi with these turbos with the stock ECU given their presumably significantly greater flow rate?

I know that back in the day M2 recommended not going above 10 psi untuned with their sequential twins - and that was with their stage III ECU.

I'm assuming you have personal experience logging AFRs above 10 psi with the BNRs on the stock ECU?

 

You would more than likely have to turn the boost down untill you upgrade your fuel system. Unless you want to blow your engine.

 

do not ever go above 10 psi on the stock ecu. plain and simple.