Running an LCD in Game mode often means you are running the panel in overdrive, which can result in a loss of image quality. Specifically I cannot say if your TV is dong this or not, but I would bet in Vegas that it sure is.
I also have a hunch the HDMI spec is limiting you to 60hz, that is unless you are using a 3D source on a 3D-TV and then the refresh is 120hz. Regardless of what marketing material may have said about your TV, what you may be experiencing here is a possible limitation of LCD technology itself. Also your source player may not support a refresh above 60hz and if you are using an XBOX OR PS3, then those frame rates are usually capped at 30hz.