Originally Posted by mattmy99

^^This
Plus:
The purpose of a retune/flash is not just to adjust fuel and timing to run more boost safely.
A tune is required/beneficial to:
-Remap the standard/aftermarket computer to compensate for modifcations.
-Modify the extremely safe factory tune to enhance/change the way power is delivered even when no modifications have been performed ie. Car is still stock/factory.

A car that is stock, aside from an exhaust, that has not been tuned will more than likely make less power than a stock car (even without an aftermarket exhaust) than has been reflashed/tuned.
Even running 98ron fuel is going to have little to no benefit (sometimes even a negative affect) in a car that has been designed to run 91/95ron and has not been retuned for 98.