I don't know about staying clean/running efficiency. When I've been watched doing track intervals before I've been told I have a good natural looking style, and don't seem to be putting that much effort in (though I feel like I'm about to die...). I was certainly very muddy by the end of Rotherham, and I'd comfortably stayed on my feet throughout. I feel quite at home on slippery mud, south Nottinghamshire/north Leicestershire where I do most of my running has plenty. Compared to the people I usually run with, I'm better on mud than road, but I'm one of the dirtiest by the finish.
I've not noticed the top runners in world cross country champs and similar magically avoiding getting seriously muddy...
