Comment 68 for bug 92599

"However, it seems to me that performance is still a little better when *not* using twinview."

This is not surprising. Regretfully compiz does have to use the detected frame rate for some (very few) things even with the fix. And if the frame rate reported by the driver is still wrong (TwinView enabled) then some things might still be slightly worse performing than with TwinView disabled. The solution should simply be setting the correct refresh rate in CCSM (Composite Section):
    Detect Refresh Rate = OFF
    Refresh Rate = the correct refresh rate of your monitor