I think I just spotted the problem with the brake pad size information you're using from Hawk. I thikn it's been staring me in the face. You're showing the HB199 and HB198, I'm thinking, for the 944? Well, per the diagrams in my Pegasus catalog (and I'm sure the same at hawkperformance.com) the dimensions shown on the drawings are for the backing plates, not the friction material. Naturally, this leads to an overestimation of the actual pad area. For the fronts it's slight but noticeable; for the rears it's substantially over. This can be seen fairly well on page 43 of the catalog at http://www.hawkperformance.com/docs/catalog_2004.pdf

My rear pad measurement, as noted, was a direct measurement of the friction material on the car (new stock pads). Calculations based on that matched fairly well with the pad area shown in the factory manual, as noted.