My guess is you have plenty of octane, and too much won't hurt anything. On my sled, I'd probably want 94+ eth-free at sea level, but once you get up closer to the 5000 foot range, 94 should be plenty, even with ethanol. 91 is the highest we can get here, but it's ethanol-free. I usually start out around 7000', and have never had an issue. Pre-mix does have a slight leaning effect. It shouldn't be enough, even at 50:1, to cause problems, but I'd have considered running in eth mode for the first tank or two. Worst, case, fat and oily could foul a plug, but it shouldn't hurt anything. I'd give everything a once-over when you get a chance: look for chipped reeds, gummed up EVs, that sort of thing. One last thing, if E10/15 is in a humid environment and not completely sealed off, it'll absorb water pretty quickly. It's hard to be sure, but it sounds like a definitely possibility. If there's anywhere you can get pure premium gas, I'd run it on that and see what it does.
