If anything, I personally would agree to swapping the Intake cam shaft and all, but I believe throwing away the Jap harness would be a bad idea! Why would you want to put in a AFM, when it's already speed density?
Here's one fact though the AEM ECU will plug-n-play into the Jap harness. So instead of spending the $1,200 (guess) for the US harness just buy the AEM.
Turbo's are fine enough as they are, you only have to watch out for boost creep if you remove the fron catalytic converter, but i have never had a problem, and I run a Jap-spec supra.
Injectors, if your dead set on using the 550's then I would upgrase to the US spec ones, with a S-AFC and some resistors.
If it was me I would run the Jap set up the way it is then buy things like, real cams, Single turbo, Fuel system etc. If you know you will be upgrading the why spend the $$ twice??
Just my $0.02