Well, I gave a snarky remark, and Kram responded. No biggie. I asked my 18yo daughter if she thought I was a jackass, and she said no. That is all that matters to me.
As I see it, there is only one important piece of information and that is does the speedo calculate speed off of the front or rear wheels. From everything I can see, it is the rear (doesn't matter which, the tires better be the same size). So, if you go with a larger tire, which will make the car accelerate slower, your actual speed will be higher than indicated by the percentage increase in tire circumference which is easily calculated.
The actual way the cluster calculates your speed basically is by taking the number of pulses (6 per tire rotation) over time and using a calibration factor based on tire size displays the speed in digital and analogue form. This method is analogous to gear ratios in an old mechanical speedometer.
There a jumper location in the fuse box with the signal that has pulses corresponding to speed that is used by the things like the radio (speed based volume), top control (so you don't open the top at speed), PSE (closes the exhaust flaps at certain speeds), etc.
That is all from me. I have a sick '01 Boxster S cluster sent to me from the land down under to fix. It is a real PITA at the moment. Probably, one reason why I came here in the first place for a distraction...