As in "the zombies run faster"? Because that would be totally imba.
In all seriousness, the Source Engine is not a particularly taxing bit of kit.
Also, with these specs on their test bench.
ntel Core i7 3930k
NVIDIA GeForce GTX 680
32 GB RAM
No one is going to notice a difference between operating systems. It might be there, but you'll never see it. The difference is 304FPS on Win7 becomes 315FPS on Linux. We only just got 140Hz monitors. The 3% increase in FPS shown here won't be noticeable. Ever.