P06 ECU in a d15b8

The ECU in our cx went out (p05) and my dad hasn't found a cheap p05 so he just wants to put in a p06. I don't think it's a good idea. Won't the fuel mapping be off and it'll run like crap? does it make a difference if we have a header? And then we'd also have to get a 4 wire o2 sensor. Seems like a bad idea. I need advice. Let me know.