A power valve on a Lt typically won't make more or less power when functioning. Its a tuning tool. What it does is trick the engine into thinking the pipe is longer or shorter depending on position.. open , being a longer low end pipe. It opens a chamber in the engine that absorbs some pressure and makes the pipe seem longer.
What It will do is smooth out the transition from mid to top end on a properly developed set up engine.
Smooth is fast.