A terrier x whippet is never going to have the speed to run a rabbit down in the open on a regular basis. But - provided it's not too big - it should have the speed to run one down in cover. As to whether or not it's too big to be useful, I think that'll depend on the size of the parents. You get both big and small whippets just like some terriers are much bigger than others.
I guess I'm biased, but having got one I certainly wouldn't want to be without one. Mine's about 15" tts and she flies through all but the thickest cover with no bother at all. And for the sort of ferreting I do, whic