THIS IS A COMMON MISCONCEPTION .....using a higher octane gas results in a smoother burn which intern allows you to safely burn more fuel....you will actually lose horsepower if the tune is not set to run the 91 at all times ....
These cars (or any car for that matter) could knock. Knock retard pulls timing (tuned or not), that in turn yanks off about 5~ WHP for each degree. If the car knocks on 87, but not on 91...regardless of the tune...the higher octane will net more gains because the PCM isn't pulling timing. Yes, higher octane will LOSE horsepower...but you have to be running some dumb high octane gas for that, think 110+. 87 to 91 isn't going to make a damn difference, hell...even ultra 94 in some places probably won't matter. Is it overkill? Yeah. More expensive? Sure. But will it kill the Cobalt you're driving? No...and it might help performance anyways. A little bit, but not anything you'd notice. It would be however more beneficial to your fuel system and valvetrain, as higher octanes...like you said...burn cleaner...and keeps things clean. I run 100% gasoline, no ethanol blended, 92 separate pump BP in my car for that reason.
The problem you get with gas these days is the ethanol drowning effect of the 8-12% gas companies mix into their gas. Ethanol normally lowers MPG's by a few percent when mixed in, hence why 100% gasoline is more expensive than the watered down crap. Another issue is not every car on the planet, especially older models are flex fuel capable...and over use of gas with mixed ethanol will cause issues down the road for older cars. Not saying it'll happen in a 2005 base Cobalt...but's possible. Ethanol is corrosive...it'll eat plastic and melt it. Seen that...fuel filters just turning to mush on the inside.
So, running E85/91 will net gains...you'll sacrifice MPG's for it, and you'll need to ramp up the ignition timing and lean the car out quite a bit to see gains from it. Probably isn't worth while either, unless you were going for an N/A HP record or something.