Its one of the most advanced rendering technique out there, like what did you expect a minimal performance penalty? There are tons of comparisons that show massive differences but you guys are always ignoring it.
But there are still big differences bewteen even Physco and Overdrive. I have finished the game with Physco RT and now im playing it mostly with Overdrive mode and i can see the difference. Sure the performance is literally cut in half almost but thats expected with tech this advanced.
And most of the time Overdrive is brighter because more lights are actually lighting up the scene and they all have bounce lighting and accurate shadows.