I did have one correction, with E90 the stoich A/F would be slightly lower than E85. A rough check:
Stoich Ethanol 9.0
Stoich Gasoline 14.7

E85: 0.85(9.0) + 0.15(14.7) = 7.65 + 2.21 = 9.8
E90: 0.90(9.0) + 0.10(14.7) = 8.10 + 1.47 = 9.6

Published numbers for E85 I have seen were 9.76. And I don't know if the stoich for the Maximal is different than 14.7 (maybe 14.5).

So, for max (safe) power, I would want to see richer than stoich, in the range I mentioned. And perhaps the dyno operator just has seen max power close to stoich on gas (but I would want to be richer to be safe). Use the cooling effect of the ethanol and put more fuel to it. But that may only be a secondary issue with the rings, not the prime cause.

Good luck.