If the car runs full timing advance on 97 you won't feel any difference on 99 because the car's got no more timing to give, depite the extra RON.
I always used to map and run my car on Esso 97 as the garage was the closest.
I never felt any difference on Tesco 99 or V-power. It was an apexi ECU so the timing run would be what I told it rather then the OE ECU being able to advance timing slightly dynmically to the max set limit if no knock was being detected.
I could probably have given the map a tweak by adding a couple of degree's timing if running on 99 and maybe worth doing for a rolling road meet, but the bumometer would probably struggle to feel it.
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