The performance difference is likely to be minimal if the shop is decent. But think about the R&D that goes into an exhaust like Fast Intentions. You get what you pay for, and there's a good reason the FI exhaust is twice the price of the custom exhaust you were quoted.
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2011 370Z 6MT Sport Gun Metallic | ARC | CJM | Ecutek | FI | Fujimura | R2C | SPL | Stillen | TWM | Z1 | ZSpeed | |