How can X possibly lose for OpenGL ? You know that OpenGL is network transparent on X right?
Years ago I watched someone play my copy of GLQuake from their machine over GLX quite comfortably. It wasn't quite as fast as a local copy of the game, but it was very playable. The VNC solution, in addition to chewing all the CPU on both machines and _still_ requiring a fast OpenGL implementation on the machine running the game, would have also chewed up all the network bandwidth, and I'm not convinced it would even have been playable.
My own OpenGL software is much less pretty than Quake, but it has practical reasons to be used over a network. Runs fine, but in VNC it would definitely incur more or less fullscreen refreshes at 60Hz. That's a LOT more bandwidth and CPU than a few thousand vertices every so often and a set of transform matrices per frame.
Maybe you have a special definition of "outperform" but for me if you needed many times more resources to get the same result, that is less rather than more performance.