I wish I got over 20 mpg... And it has an estimated EPA highway mileage of 28mpg and 18mpg in the city.

I also live in Wisconsin, and your milage is fine. Much better then mine was. My last fill up I got 16.6mpg

at 70 mph 25-27 mpg is what I get. And by the way I am sure that the EPA estimates are done at a way lower speed than 70 mph, probably more in the 55 mph range, a speed at which I get near 31 mpg.

premium will not increase your mileage unless you are getting pinging currently
you should still run it though

This is not entirely accurate. The knock sensors will not allow any pinging by retarding the spark. So you will probably never get pinging that you can hear. However, retarding the spark will have a major effect on fuel economy. Running regular in a supercharged 3800 will almost certainly require some knock retard therefore effecting fuel economy.

I have even noticed a difference in my normally aspirated SE by switching from 87 octane to 89 octane. I have measured a difference of around 1 mpg on long road trips.

what i said is still basically accurate. the knock retard prevents ping, but the end result is the same. unless it is detecting knock, and retarding timing, it will not increase gas mileage. an engine with 0 KR on 87 will not benefit from 89 or higher