i think a lot of the difference is right there. You may buy a 3inch exhaust system, but if it isnt mandrel bent, then you may end up getting an exhaust system which is only 2.5 inch in size at a certain point -> which will impede airflow compared to a 3inch mandrel bent system.
I had my fujitsubo 3.5inch exhaust fitted at an exhaust shop, and the guy commented on the quality of the tig welding, mandrel bending and the stainless steel used on the system. Basically, he said that NOTHING made here could match that sort of quality for the price. So while local systems may be cheaper, your jap gear may be of better quality (depending on the system you buy)