I did 20". Feels and looks great to me. I wouldnt do anything. My new height is 11'4". Ill keep my clearance to under 12. Personally, i think i cut my roof in the best possible spots, for a conventional, bluebird body. Only thing I would do differently is used galvanized sheet steel for the new sections. thats what the factory used. Depending on where you drive, clearances will be important to you. Even though im in the spacious west, I think staying under 12' is important. Most coaches do.
Patina enthusiast and professional busman