Exhaust Restriction

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I've done a search but can't get a conclusive answer. I will be getting a hayabusa within a few weeks. It's a 2003 with stock exhaust system and stock pipes.
Does anyone know where the greatest restriction lies? Ie is it the stock pipes, the mid-pipes or the headers? Eventually I'll be looking at getting a 4 to 1 on it, but money will be tight for the next few months.

Let me know when u are after a 4 in 1. I sell TiForce systems, they weigh in at approx 4.8 kgs for the Gen1, and u get approx 10+ rwhp.

Ray, I thought Gen 1 TiForce was 3.75kgs?