The 996 TTs only ran on 18" wheels, they never had OEM 19" wheels on them. I'm not sure what the offsets are (I used to own a 996tt) but I'm sure they won't fit without spacers. If you search through the tire and wheel forum, I'm pretty sure this question (996 TT wheels on a CS) has been asked and answered before...
in fact the thread I'm thinking of is listed below under "similar threads." Always take a look at the bottom of the page for these...
http://www.caymanclub.net/cayman-tir...heels-fit.html
brad