I ski at 4500' with a 6.0 200 with 1800h, still strong WOT 5500/43mph, the only gas available here in Brasil is 27% ethanol. Besides the normal maintenance, ie plugs, cables, impellers, oil and filters, I only changed an air sensor, water pump and the rpm gauge.
I also ski with 5.7's MC 190, Response LXI and a local MB boat. All those 5.7 boats are terrible to ski due to the lack of power and consequent speed variation at 4500'.
I don't think a better octane fuel would change anything, as I read here before, HP, love and money, the more, the best.