I installed mine as well. They are much lighter. Specific weights are:
OEM STi Rotors: 22.415lbs
KVR Rotors: 16.275lbs
This matches the weight savings in my wheels (12.7lb SSR Competitions), though saving weight in the wheels is better since the weight is in a more important area (further out on the circumference). I also am trying the Ground Control Coilover kit which saves 4.6lbs in each front strut and 4.2lbs in each rear corner. 16+lbs total savings per front wheel and 10lbs per rear is nice! All told, this is a 52lb savings, all unsprung and much rotating/unsprung.
One reason these rotors are lighter than OEM ones is that the metal in the disk's wall is thinner than stock. IE, while they are the same thickness, the KVR rotor has more venting space inside.
I'm not sure which is better on the track: more mass to absorb heat or more venting area inside to aid cooling. Track time (this weekend) will tell.
KVR Venting thickness: 15.5mm
STi OEM Venting thickness: 12mm
Here are comparison photos.
Note the difference in space in the center.