Jim, have you read the most recent comments from the PCA web site's tech area on this subject?
PCA Article on 18 inch rims on 97 boxsters...
Joel Reiser ended his explanation with this:
"All that being said, I would not be too worried about running 18" wheels on a street-driven 1997 Boxster as long as the widths and offsets are in range with what the factory offered later, and you are not driving the car to the limits."