It shouldn't thin the beer out from where it's at now, but it will add alcohol without any extra body or mouthfeel. A brown ale already has a good amount of flavor going on, in most cases, and I'd expect the honey will not add a significant amount of honey flavor or aroma (depending, of course, upon how much you add). The effect would be subtle, unless you get over 3-4lbs, I'd want to say. That would add a good amount of fermentables to your wort, though. The best thing to do might be to find a fresh honey with as big a flavor and aroma as you can find (some real strong variety, or maybe a blend), and use a smaller amount of that, like 2lbs or so. You can always add more, but you can't take out what you've put in.