Is installing a tankless water heater outside a good idea?
Are you OK with "beautifying" your house by adding a post-industrial or modern "style" water heater on the outside of your house? Is there some other interior hidden space close to the bathroom that will be less conspicuous? 

You could also run a dedicated ½" or ⅜" PEX hot water line to the bathroom, which will store much less water and therefore result in faster hot water there.