Depends. I lived in a place that had a tiny bathroom with a 15' ceiling. I wanted to loft it. 

my apt has a bathrrom like that. 7 ft long, 3 ft wide, 9 ft tall. toilet centered on the end opposite the door. love it, feel so powerful sitting there. like a Phoenician king.

My old house had a washroom with a sloped ceiling. 9ish feet at the short end and at least 12' to14' at the other.

Loved it.

Unless there is some old-style division in the wall surface (via finish, color, trim, etc.) it will feel very uncomfortable. But with proper treatment of walls and ceiling, it can work well.

I can only imagine this working in re-use situations, where you're inheriting a ceiling height (for example, a school to apartment conversion). Otherwise I just don't see a reason to do it deliberately.

Canyonesque bathrooms can be fun sometimes though.

My 2 cents

The dimensions, scale, and shape of the space are determined by what you are trying to convey.

agree with z1111... but in the vein with gruen, creating those proportions with the height will make the width and length seem considerably smaller, something that may have worked in a lower ceiling space may not work or feel right here