Ever played BLOPS on PC? Especially on lower-class PCs it is simply terrible performance.
I recently updated to an i7-2600k with 16GB RAM and a new 2GB Radeon HD6870 and only now can I experience it entirely stutter-free (on highest settings).
MW2 has always run so fluidly for me, even on my two PCs from before, and the older one was utter crap....
So I don't really know what you're getting at, the MW2 engine as well as the MW3 engine (which are basically the same) run so much better than most FPS engines anywhere!