Well from what I remember it has built in resizing, etc. But then AVISynth is probably faster as it resizes in YV12 for a start. Newer versions of CCE support YV12 as input but as far as I know they just internally convert to YUY2, so presumably it does that before resizing.
Doesn't make a lot of sense to me YV12-->YUY2-->Yv12.
The only MPEG2 encoder that I know of that supports YV12 input by the way is libavcodec which isn't a bad encoder by the way.