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Steve, the stroke of the m/c is a red herring IMO. You are quite right about the comparison of the Type 1 and type 3 calipers that show that they both are fed by the same m/c and that the Porsche calipers fall as a size between them. Ergo the stroke cannot be an issue. The 'normal' (if there is one) clearance for the pad to disc contact is 14 thou (0.3556mm) so you can calculate the amount of fluid to take up the clearance. Thereafter the amount of pad/fluid/seal compression is variable depending on the pad material, fluid type, seal type and not forgetting the hose expansion. It is impossible to predict accurately therefore how much the piston will travel - it can only be determined by trial and error.
Another point worth mentioning is the difference in pad sizes between the Porsche and the VW pads. Assume that the Posrche pad is 20% larger than the type 1 pad and given by your figures the Porsche piston area is 5.25% larger then the compression force/unit area on the Porsche pad is LESS that that on the VW pad for a given pedal pressure. Therefore, a) the pad doesn't heat up so much so as to soften the material and b) there is less force to compress it.
Think on this: Alfa Romeo developed their 116 V6 Rally/Circuit car that was homologated with twin standard calipers at the front and the standard single caliper at the rear. The vehicle is also homologated with a standard dual master cylinder of equal 25mm pistons for the front and rear. Doubling the piston area at the front had little untoward effect on the braking or driving ability of the car and excessive pedal travel was definitely not an issue. Also given that the rear m/c piston had to travel 4 times the length of the front piston as the difference in caliper piston sizes front/rear are about 2x in standard format if m/c piston volume had been an issue it would have been profound in this set-up.
It seems to me that your checking engineers don't know enough about the theorey of braking systems and appear to be applying illinformed and unfounded intuition. God save us from amateurs!
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