I had L4D2 on Windows so the Mac version was free and I installed it.
I am running it on a new i7 iMac and I would say that the performance is right on par with windows, possibly even a slight advantage.
For comparison, I run Windows 7 via Bootcamp. Both settings are identical on both OS's (thanks to steam, love that!). Native resolution which is 2560 x 1440. Anti-Aliasing Off, all detail levels high, antistropic filtering at trilinear, I've tried higher but I just cannot see the difference, ymmv.
On average, 45-60 fps at these settings in both OS's. I think in heavy scenes the Mac has a slight advantage but that could be a seat of the pants rating.
End opinion, if I ever have to re-format my bootcamp install, I won't bother re-installing L4D2, it runs just fine on OSX.
troutspinner