I'm no expert on /5's, but have read a LOT of tire threads here, and I can tell you that "fat" isn't always a good idea for these bikes. They may actually handle a lot better with the "mountain-bike" tires that came standard. . .except now the compounds and tire construction are much, much better versus when the /5's were new.
We've now become accustomed to seeing "fat" on the newer bikes to the point where "skinny" just looks plain weird.
Think of the poor squidly astride a new 600cc rocket -- just passed in the twisties by your cafe, going like a striped-assed ape and riding on those itty-bitty tires. Hey, that alone is worth staying with stock sizes!