To update this thread, the wheels fit just fine on the car. The mounted tires are another story. I bought this set of wheels/tires from a person that had done one season of autocross on them, in a Cayman. The Cayman (and presumably 987 Boxster) run a slightly taller tire, about 1/4" taller. The set I bought had slightly larger tires, around 1/2" taller. These taller tires fit in the back, but it was close. In the front, the taller tires would not fit, the top of the tire was firmly pushed against the "cup" on the strut that holds the bottom of the spring. I don't believe it was a spacer issue, as the stock tire goes in about as deep.
Maybe stock sizes of Cayman tires would fit on a 986 Boxster, but I bet it would be very close in the front:
Stock 986 Boxster front - 205/50/17 - diameter 25.1" (from Miata.net tire size calculator)
Stock 987 Cayman front - 235/40-18 - diameter 25.4"
My larger front size - 245/40-18 - diameter 25.7"
The backs looked so good when I put them on
But it was a no go in the fronts.
I may try and take the tires off and sell them separately as I have always wanted a set of the 18" Cayman S wheels, or I may try and sell them as a set for someone that tracks a Cayman or 987Boxster.
After all my calculations about offsets, etc... I never considered tire diameter as being an issue. So I learned something yesterday.
Steve
PS I never did let the car down off the jackstands so maybe it would have been the case that once loaded these tires would have fit. But the way it was pressing against the bottom of the cup that holds the bottom of the spring, it seemed like being loaded would have no effect or possibly make it worse...