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I believe that the sodium filled valves, used in Z06 Vettes, which are lighter to allow higher revving were longer to offset the smaller base circle of the LS6 cam.
How might lighter/longer I don't know off the top of my head.
Just the valve weights. Ideally you want the lightest valvetrain as possible but there really isn't enough difference to justify chasing after a head with those valves IMO.