Smoke ring stops at 140, but meat will continue to take on the flavor continuously. OTOH, I read a tip from Todd of Plowboys, or maybe Rod from PE--someone of that stature, and that tip was "try to undersmoke your meat!" I'd do light hickory + cherry for about 4 hours myself. Sounds great for pork butt! I've gotta get some PECAN!!!!! And another brisket!
Yeah. What you said.