Originally Posted by ChrisSlicks

2-piece rotors might give you a "slight" acceleration boost, I didn't notice anything striking. Only my front rotors are significantly lighter, the rears are within a pound or two as they have iron hats.

A while back Grassroots Motorsports did a laptime test using different weight wheels, the times were all extremely close. In fact they noted a higher top speed with the heavier wheels (the rotating weight acts like a flywheel does).

Forget lightweight 1-piece rotors, those are not suitable for tracking. You are just reducing mass that can absorb the thermal energy from braking.

The real question you have to ask yourself before buying 2-piece rotors is how much is it going to cost in the long run? I have to replace my rotor rings after every 12-14 track days. Theoretically new rings should run about the same as new 1-piece rotors but you are at the mercy of the vendor. Make sure you find out the replacement cost before buying - usually new mounting hardware is required every time as well (hat to ring screws, nuts and washers). For me the cost is an acceptable trade off, but do your homework first.