Newer engines, especially variable cam engines can really benefit from higher octane fuel, even if it's tuned for 87 (factory). Especially in warm weather or towing. The ECM constantly monitors the knock sensors and will retard timing at the slightest signs of pre-ignition. If you watch the data parameters on a scan tool the higher octane fuel will create less "knock" and therefore less retarding of ignition timing. More advance = more efficiency. so it's my opinion high octane fuel may not gain you a noticeable power difference, but will allow your engine to run at optimal timing without backing it off, and that's what will give you optimal fuel efficiency. of course higher octane really only comes into effect in warmer temps and high cylinder pressures "towing or hot rodding". Otherwise your engine will likely not record any knock retard. the EcoTech 3 - 5.3l has incredibly high compression (11.1 or so I believe, because Direct injection delivers cooler fuel) and you can take advantage of this compression by not giving the engine any reason to retard the timing.
Personally I have experienced 1-2 mpg gain by using 91 vs 87 on long highway trips with my L83 Silverado and Suburban. In town won't really make any difference. And definately stay away from any form of alcohol (E10, 15) unless you have a flex fuel engine that will actually read alcohol content and adjust the fuel strategy. Non flex fuel engines will only see the lean exhaust caused by the ethanol and richen the F/A mixture because it doesn't know any better = lower power and mileage.
Flex fuel engines will actually see an increase in power from the ethanol because it reads the alcohol content and can advance the timing to take advantage of the high octane and cooling effect of alcohol in the combustion chamber. Unfortunately ethanol will never equal the mileage of 87 because of its 30% lower BTU rating; so 30% more fuel is needed to reach optimum fuel air ratio.
I have heard if an engine is designed specifically for E85 it would be incredibly efficient and powerful by taking advantage of the high octane rating and low cylinder temps so it can run high compression ratios and high ignition advance. Unfortunately these engines are not mainstream and I don't believe offered as factory engines because they cannot run on gasoline and require at least E85+. I think you would need a custom tune, as well as specifically designed intakes and combustion chambers to take advantage of alcohol as the primary fuel.
Not an expert, but just what I have experienced and noted watching scan data and hike driving my own GM light trucks.
