If he's going to use the CX harness, he'll still have to remove the JDM harness and swap it on, so I don't really understand why you think it will be easier than getting a GSR harness.

If he wants to use the CX harness, he has to:
-remove D15 or whatever is in a CX
-take off CX harness
-take off JDM B18C harness
-put on CX harness
-lengthen some wires

If he uses a USDM B18C1 harness:
-take off JDM B18C harness
-put on USDM B18C1 harness

Of course he still has to take out the D15, but swapping the harness is something he can do prior to the big swap day, and it's two steps less than using the CX harness (at the cost of an extra harness).

Maybe I'm just not understanding what you mean, but I don't see how it's harder to use an extra harness.