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I would never recommend mailing in your ECU for a generic reflash unless you just need something simple like removing the speed limiter or raising the rev limiter.. but you can see good results with etuning-- you get your own cable to reflash and going back and forth with a tuner adjusting the tune with datalogs that you record while driving around. Still cheaper than a dyno tune and you get the cable so you can reflash back to stock and sell it used later on, reflash updates if you need any, as well do all kinds of OBD2 diagnostics like checking and clearing codes
If you can (me in Italy for example i had no choice, no tuner for this car and uprev software...) it's better DYNO for sure for clear reason, but also emailing it can be quite good if you use logs system and NOT a generic tune !