Even with a dedicated line, a Wattagate 381, & upgrade AC cords: I've still realized sonic benefits from line conditioning to my amps. In addition to the VansEvers & Shunyata, the Chang 9900 provides beneficial noise filtering without constraining dynamics. I also use the low-level Chang 3200 for my source components.
Raising AC line voltage would slightly (but insignificantly) increase maximum available power output from an amp, although dynamic drive capability could certainly improve. However, the step-up transformer will constrain the dynamics so that seems self-defeating?