Even a small difference in the density of the air charge can make a significant performance difference. I once installed a better intercooler in my Buick (dropping air charge by about 3 deg C), and picked up 1/4 second on the G-Tech!

As temperature and humidity both fall, you SHOULD notice a change. Temperature for the obvious reason: denser air. Humidity because the more water molecules per gram of air, the fewer oxygen molecules, hence more O2 at a given density.

There's also a seasonal increase in O2 content - that's really slight, but again, it doesn't take much.