I graduated from Kellogg just a month ago. A lot of people at Kellogg told me to waive the micro class if I could (I may have been able to), saying that it was dull, but I actually thought it was one of the better classes that I took at Kellogg. Besanko, who's a tremendous teacher, obviously had a lot to do with that. But I also enjoyed it a lot in retrospect because it was such a fundamental building block of many of the other courses I took, many of which were dedicated to strategy. I had taken micro in undergrad and thought, "Yeah, yeah, supply meets demand, Pareto efficiency, blah blah blah," but there was a lot of more higher-level thinking stuff that I just didn't get (or maybe just didn't appreciate) the first time around. Anyway, I definitely recommend taking it, even if you can waive out of it.
Good luck! Don't worry too much about the GMAT. I mean, obviously you you want to do well, but no one at the top business schools is remarkable because of his or her GMAT score alone.
Scott