In most cases, on two stroke engines, yes... the diverter will rob a bit of power.
Best way to prove it to yourself is to fly it with, then without the diverter.

My personal experience has been that it doesn't rob enough power for me to worry about it, and
if I have a plane that can stay a little cleaner, I'm not too upset giving up a couple hundred RPMS.
If I were flying some scale-powered or underpowered planes, I might feel differently.
If I was flying a speed plane, to go fast... I'd ditch the muffler anyhow, and go with a tuned pipe.