I, too. like more smoke than most. There's no harm in adding some more if that's to your taste as well. 'Over-smoking' is a matter of perspective to some degree, but there is a point where the smoke taste can overwhelm the taste of the meat and/or the rub. Where that point starts is open for debate but I think at some point on a continuum, were you to taste progressively smokier meat samples, you'd arrive at a sample where the smoke became virtually all you'd taste.
Imo, butts are harder to over-smoke (large, bone-in ones anyway) because of their size and thickness. I think at least some people could find a butt over-smoked (just one teeny piece of wood would be too much for my Cuban friends down the road), but since you like more smoke than most anyway go for it. Worst case scenario, imo, is that while the interior would be fine you might lose the taste of the rub. One thing to note though: mesquite, like black walnut and a couple others can taste bitter if excessive, even before the point of 'over-smoking'. You might want to add a lighter or sweeter wood.
