OK, I have an idea for a simple test for those who are concerned or interested in trying it. You might need a couple hours to do this, but it should work out to track your battery usage.
Charge your phone to 100%. Set screen to 100%, turn on "best wifi performance", and stream something (audio or video) for one hour. Make sure nothing else is running (facebook, or any other apps that might screw with wifi or CPU performance). Keep the phone in the same place so you're not changing the distance from the device to the router.
Make a note of your battery life after one hour (or go 2 hours if you want. Or stream one movie)
Charge your phone to 100% again. Keep everything the same (no other apps running, screen brightness, distance to router, etc). Stream the same movie, or stream another hour of audio. Do your best to keep everything exactly the same as before -- the only difference is to turn off "best wifi performance". Check your battery after one hour, or whatever timeframe you used for the first run.
Heck if you have time, repeat this test (both parts) a few times. Keep everything the same as much as absolutely possible, only toggling this "best wifi performance" option on and off.
If you keep everything else the same, doing a test like this (especially if you can repeat it on multiple days) should give you a clear answer as far as how much battery/cpu is being used.