Random advice. I'd go through a couple of tanks of decent fuel. In UK, Shell V or Tesco 99. Not because of octane really, more to do with the additives that keep the bits cleaner. Made a world of difference on my other car which is a V8, not a performance engine, injection, basically just a big slugger. I only go 4 miles to work, the Camaro does about 16mpg, my 2.6 Omega does around 18, but is hugely better on longer runs like 35 or something. If your in traffic they all guzzle. Reset the ECU if you can. I don't know specifically about short cuts on Omegas but generally with GM, just disconnect the battery for 1/2 hour does it, so not assume 31 minutes is OK, leave it off for a while. I wouldn't short the battery connections, you might save 31 minutes but you don't know the the condition of the capacitor.
16 - 19 in traffic depending on your driving style doesn't seem excessive to me. Try it on a longer run but don't try and fix it if it's not actually broken :-)
T