For higher hydrations I actually prefer faster mixer speed and or longer mixes. For example roman pizza uses a very high hydration dough that is typically mixed in ultra high speed spiral mixers for 20 minutes or more. I agree with craig, though, and above 70% you are probably going to have better luck with a spiral mixer or just doing stretch and folds by hand.
My issues with the santos came when trying do do 59-60% hydration doughs. Some people may not realize this, but thats the hydration range at most pizzerias in naples and even many of the premiere pizzerias here in the US such as Keste, Amano, Don Antonio etc. There are a few places doing higher hydrations.... Da Michelle is closer to 65% for instance (I forget the exact number now but it was eiether 64.5 or 65.5), but the norm for Naples is lower. With 59-60 percent hydrations the santos tended to develop the gluten very fast and even when I compensated with techniques to offset this I found that the resulting doughs were not as good as what I could get out of other mixers. Im not the only one that felt this way, and there were even forum members that came up with mods to slow down their santos (hence the pietrosantos) . I could be wrong, but I think Bill tends to use hydrations more in the mid 60's and above, and at those hydrations the santos at its normal speed worked fine for me.
The inventor of the pietrosantos told me that slowing down the mixer actually made a better dough at any hydration, but ultimately he still had the best results with hand mixing.