I'm not Cornus (as you know) but yes you can prune now altho it's not the ideal time for our area. Can you wait til February? Just before growth starts and your yew is not settling down for a long winter's nap. But you won't kill it if you do it now.

It's been my experience that you can cut back a Taxus as hard as you want and it will grow back. I have one that was once over 12 ft. high which I cut back with a chainsaw to about a foot. I was going to dig it up and move it. I never did and it has been sheared back ever since to about two ft. tall with no ill effects. This has been going on for over 25 years. I'd move it, but other roots have invaded the rootball and it would be hard to move without doing damage to neighboring trees and shrubs.
Even in your climate, I can't see how you can hurt it by cutting it back. I say, go for it.
Timing? Anytime. If you do it now it will look bad until Spring though. Yews are almost indestructable.