Just a few data points driving my 5.3L flex and recording engine data with my Tech 2. With E10 87 (Costco), driving around town, my engine has knock sensors going off all the time with correlated timing adjustments. I have concluded this is how the engine is tuned, to "ride the knock sensors" for timing adjustments when running 87.
I haven't tried E10 91 yet, but I did run a tank of E85 through (it was actually 80% Ethanol, which has a very high octane rating relative to the E87-E91 range) and the knock sensors did still kick off once in a while, but much less.
I will try running the Costco E10 91 next and maybe post the two snapshots side by side for comparison, but I suspect the knock sensors will be more active with the 91 than they were with the E85.
It would be nice to have one of the guys who have a 6.2L and a Tech 2 post up similar results. I suspect even with 91, the engine will be riding the knock sensors some, if my results with the 5.3L is any indication. I definitely wouldn't ever consider running 87 in the higher compression 6.2L.
The caveat I would offer is my engine is high time, so likely is more prone to knock than a new engine with less build-ups, etc.
My opinion on the 6.2L and the additional cost of Premium is, I wish my engine was a 6.2L.