Here's two of the 12.5" X 1.1" rotors from my SHO Baer brake system. On the left is the full Iron Kelsey Hayes rotor, on the right, a 2-piece unit built for me my Todd at TCE Performance.
Click thumbs for big image.
2.71 pounds difference, so the 2-piece rotor is about 16% lighter. For an 11.6" rotor, I would expect the poundage difference to be less, but the percentage difference to be about the same. I wouldn't call 16% less particularly dramatic when thinking about its barely noticeable effect on a heavy street car. Additionally, the reduction is all in the center of the rotor, just the hat, so it's effect on reducing "rotating weight" or moment of inertia effects is negligible. And on top of all that, these aren't going to stop the car any better than a one-piece rotor.
Now 2.7 less pounds of unsprung weight on the suspension assembly? That's nothing to sneeze at, but again, for a heavy street car, is it worth the extra $Hundreds for 2-piece rotors? Eh, only your wallet can answer that.
If you want 2-piece rotors for bling or to grease some rotor envy issues, hey, knock yourself out, but satisfying this urge is going to cost you some serious coin.
