Youre mixing up a few things, terminology as one, e.g. antilag is retarded timing and additional fuel, resulting in a lot of flame going into the turbine causing boost, there is no hesitation with antilag. My issue was the fuel injector cutoff kicking in very quickly, which ive since resolved. Im also turbo, so theres a tonne of other things that can can issues lol
The USA dont have the same grade fuel as us. Same go with the Japanese fuel, however all japanese cars are tuned (high performance ones anyway) to 100 octane, which on aus 98 encounters knock and thus retards.timing a couple of degrees. The USA 91, while is often compared to our 98, is graded differently, and unless you get a vial of the stuff through customs to check yourself I highly doubt they're close enough to take their tuning advice willy nilly.
Grab a dyno, grab a knock sensor, and tune to mbt on aus 98, then then you'll be have a tiny tiny kick of power while NA.
Every engine responds differently to tuning and timing changes. Mine might love +6 on 91, but yours might struggle with +2 on 98. There are so so many variables with it, from bearing clearances in micros, oil contamination, fual grades, fuel flow, electrical noise, piston carbon build up, ring gap, spark plugs, and so much more.