Performance mods, I have done nothing, yet (have headers waiting in the garage) but I put new plugs/wires/cap/rotor/o2 sensor/pcv valve in last winter. It's an automatic, and I don't drive it like I stole it, and I use the cruise whenever I can.
I don't think so, maybe in the winter we have the oxygenated gas or w/e its called so it doesn't freeze. I know I usually get worse mpg in the winter months. The only time I noticed a difference was when I was in certain us states and I mistakenly put 85 octane in, instead of 87 which is the minimum. It actually made a pretty big difference and the CEL came on. Then again it was probably used to my elevation.