What patterns are you using to measure grayscale and brightness? You should be using windows or maybe APL patterns with a plasma. What issues are caused by setting the contrast higher that can't be fixed with the W/B controls? Both 80 and 110 cdm2 are fairly low for their respective applications/room conditions IMO... I would think that you should be able to go higher without much trouble, but I don't own that model. I usually like around 120cdm2 for a dark room, but everyone's different I suppose.
I think the general rule is to not so much target a specific brightness, but to set the contrast to a comfortable brightness level for your room conditions, as long as there aren't any negative effects, such as white clipping or color shift in white levels below ~240-242. That's what I do anyway.