mate, you need to read up on the properties of ethanol. HPI magazine (Martin Donnon) ran a good article about the dangers of thrashing a performance car that was not tuned to run on fuel containing ethanol.
But once the GTR was tuned then it made more power everywhere
If the fuel was 100octance with no ethanol, then you would be right. The existing tune would be fine, and the higher octane would provide a effective buffer against detonation.