There was a time, a few months ago, where I thought I was going to get a used 997S and was buying a lot of Porsche magazines like GT3, 911, and Excellence.
I cant remember which magazine I read the article in but there was a comparison between the Caymen and I think old 78 Porsche RS. In the article they actually put both cars on a scale. The results for the Caymen actually surprised the Magazine and if I remember right the weight came to over 3300lbs. The whole reason the comparison was done was because the RS and the Caymen were boths supposed to be real close to 3000. I think the RS was actually less than 3000.
Anyways, I wouldnt be surprised if the GT3 is heavier than what Porsche clames. The difference is probably closer to 400lbs. That is if the M3 is not heavier than we expect.