There is a hit. Will get solid numbers later. There are significant differences depending on how many light sources are on screen.
Few other things:
1) So far, I've found candle/torch production to be the biggest bottleneck, thus my preference for braziers and fireplaces.
2) Water and lightsources have an interesting relationship. Underwater trees still produce light, but drowning a brazier removed the light. Have not yet tested with gems.
2a) Underwater trees look really cool. I'm hoping that future versions of rendermax will allow lightsources to illuminate a multi-z-level sphere. Would make the underwater seas look awesome.