here's a thread;
http://986forum.com/forums/general-discussions/34205-cayman-s-exhaust-boxster.html
there's some misinformation in there, but berty and todd holyoak have the facts - it will fit, it is lighter, it requires that the whole system (headers, mid-pipes and manifold) be changed, it may require extending your post-cat o2 sensor wires, and go for a take-off from a 987/987c 's' model. it sounds better and should be good for a few hp (hey, 10 lbs = 1 hp, right?).