The answer is yes, changing your tire diameter / width / profile will affect the accuracy of your speedo. The effect is proportional to the change in tire circumference (Pi times the diameter).
Just in case your mathmatically challenged - here's a handy calculator to determine the affect of changing tire size:
http://powerdog.com/tiresize.cgi
In your case, the change is small. In fact, I have the same size tires and part of the reason I went this way was to correct an over reporting speedo, i.e., it said I was going 80mph when I was really only going about 76-77mph. I am not aware if it is even possible to recalibrate the speedo.
Jim
97 tt BPU
P.S. How's Madtown? I went to school there in the '70's and would love to get back!