[racer]

The boxster wheels should be 7" front and 8.5" rear

A tire size breakdown:

205/50-17 means:

205 = tire width in mm
50 = aspect ratio. ie, sidewall size as a percentage of width. 205 x .5 = 102.5
17 = rim size

if you do the 255/40 math, you get 102.. so the tires, as mentioned have a nearly identical overall diameter.

I would suggest a 225 width front to help combat some understeer.

I have a similar current set up (225/45-17 and 255/40-17) on my 8V N/A and am actually looking to go 225 all around to help balance the car a bit better. imho, the 255s are just too much for the car.