nope, GS300 doesn't get better mileage than IS. I can tell you this because my dad has the GS and I have the IS, and I am getting at least 1 mpg better than him, and he is the more conservative driver of two.
Long ago, I posted a link to a URL that describes how the EPA does mileage testing...and get this, they don't actually measure the volume of gasoline consumed. Instead, they measure the emissions from the exhaust, and I guess there's some formula to figure out the mileage. How wacked out is that?!