whats the difference between a JDM b18c ecu and a USDM B18c1 ecu?

 

better fuel curves and air ratio

 

JDM ECUs usually don't have the ELD, or a few other things, and there are the differences between the two engines maps, and because they have different fuel/emissions in JP.. GSR and ITR heads are different, different rev range, cams, compression, dual runner manifold, etc.
Because you're talking about GSR/ITR ECU differences, the ITR ECU won't have the IAB (secondary butterfly) circuit that the GSR ECU does. Depending on the year, there are immobilizer differences also, if that applies. I can't think of anything else, but I'm sure I left something out..