They don't really put any vacuum in the crankcase at full thottle, because there is no (or almost no) vac at full throttle, so they probably don't increase max power.
At a light cruise though, the PCV valve could very well be open and helping your rings seal better due to vac in the crankcase.... theoretically....
I like to run them, though, except on an all-out race engine that would never even produce enough vacuum to open them. But they help keep your crankcase clean by sucking out oil vapors and condensation. This keeps crap out of your oil and deters sludge buildup. And they don't really hurt anything; you just have to richen the idle mixture a bit. It's a win-win.