Mine ran MFi and NO 5th Injector at 250bhp. Infact I have never run a 5th Injector on my car. I went straight from a Standard Configuration 4 Injector MFi Setup to EFi.

In answer to your question, I would keep with the 5th Injector as it is. It is quite crude, but at least it allows for sensible off-boost and low boost fuelling with increased enrichment higher up.