I would never recommend E3 plugs to any vehicle, its basically a spark plug to get you from where your stuck to home, then toss out. I would recommend what OE uses, the AC Delco Iridiums, best plug to last you 100k mileage, averagely.
For 14mpg, do you have any smell of fuel odor after the vehicle has turned off? Checked the air filter or notice any unusual idle?
2009 Cobalt LS XFE U74
Mods: K&N Typhoon Intake, Option Cat-Back Exhaust, Cosmo Short shifter, Maxxim Ahead 17X7's, MPx shorty antenna, FE5 Strut/shocks, Tein S Techs, Moog endlinks, Moog spring seaters, ZZP shorty header, LSJ downpipe, LNF front grille