I bought my saab late this summer. MPG has never increased or decreased... It has stayed more or less constant, the biggest factor being traffic jams. I get ~34 mpg, driving at about the same speed as you do, shifting at about 3000, and most importantly, running on NJ gas. I havent seen much of a idfference overall between summer/winter gas, and believe theat NJ offers 'winter' (oxygenated) gas all year round now. I had a tank or two bought in PA that yielded close to 40 mpg on an all highway trip, however that was constant cruise at about 71 mph, on the PA turnpike. I cant say that it is because PA gas actually has more 'gas' and less oxygen in it.
In my pickup truck, I have gotten the same mileage since I bought it new as well. In fact, I cant think of any one of my or my parents cars that actually saw a mileage boost as it broke in. We all always run our AC units though (never had a leak, bad seal, or improperly working AC system, so I guess the fuel loss pays off).
Lighten up the foot if you want to get better mileage. Its not that you cant drive at 75 mph or whatever, but just take a little bit longer to get to that speed when you can. Your car and wallet will thank you.
JMH