I get what you're saying now about glxgears - it's probably an old (bad) habit I have from when I started using Linux in 2005, as a way of checking whether DRI was working (the good old days!
I see what you mean about the fps figures being unrelated to the lag - the current 295.33 installation is giving approx 750fps in glxgears, yet the GUI is as responsive (if not better) than with nouveau. I admit it - a facepalm moment! As I said above - a force of habit, rather than a useful comparison.
As an aside, when did the '--i-know-glxgears-is-not-a-benchmarking-tool' (or equivalent) switch become the default action for glxgears?
On a final note, I'm happy either way - the GUI is responsive, and 3D apps are now working!