Honestly, the MTB cassette is overkill for road riding, I problably use only the 6 smallest cogs on the road, even on steep hills. Of couse I'm more of a "masher" than a "spinner"
For CX the combination is usually pretty good. I say usually, because last Sunday, the mud was so thick that I could hardly turn the cranks on the flats and even had to pedal to carry any speed on the downhill sections. I was really wishing I had brought my MTB to the race that day.