From a lot of experience with motorcycle exhausts, some experience with car exhausts, and more recently with my Plus 4 including a lot of work on the ECU calbration, I would say that you would definitely need a re-flash to get the best out of it. I would not worry about the drivability of the car, you will probably get an improvement across the range. It is not as if you are changing the cams or size of the valves etc which could have the effect of shifting the power band.

Peter

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