Valvetrain weight vs. RPM limit

Force, inertia loads, and friction increase at the square of RPM so component weight will become a greater factor at higher RPM. Lighter is always better within the limits of strength (and budget). The Comp "beehive" springs allow a substantially smaller and lighter retainer for example. Roller rockers are sometimes (not always) lighter BUT they move their mass outward, over the valve tip somewhat diminishing gains. Roller tips are great for valve guide life however.

Lighter valvetrain components will allow an engine to rev a little quicker, but not nearly as dramatically as lighter pistons.

The advantage lighter components have power wise is they allow less spring pressure for a given RPM. Think about it, it takes horsepower to open those valves against stiff springs. Less spring pressure means more power to move the car. Less spring pressure means less friction at the lobe/lifter interface, and therefore less wear. Up to the point where you are limited by valve float, too little valvespring is generally better than too much.