As far as being within the working range of the 6500 derailleur, I'd say it will work. Shimano specifies that a 27-tooth cog is the upper limit for that model, but they're pretty conservative in their spec. My concern have more to do with the entire gear range of the cogset you're replacing the 12-27 with. If you're going to use an 11-28 mtb setup, you'll have some widely-spaced gears on the low end. Also the difference between 27 and 28 isn't all that much. It probably won't feel like an "extra gear" when you're climbing that monster hill.