Be careful, FFdshow doesn't use multithreading (well, it sort of can, but by default it comes disabled), so the moment CPU usage hits 50%, it'll slowdown.
Don't take this as an insult, but I'm quite sensitive to framerate fluctuations, and since on that machine it shouldn't be terrible, perhaps you aren't noticing. You can press control+4 in MPC to enable the statistics and check that the Jitter and Sync Offset stay at 0 (or at least low and constant), specially during the first seconds.>>11086113
Wouldn't make a significant difference unless there are other problems. In this case, no, local hard drive.
The point is moot though: CoreAVC played it better, so it's an improvement. Whether FFdshow is fast enough is another story.