From reading about the antics of drug smugglers, they seem to like flying at low level, which would mean that 2 engines would be operating at low power (inefficient), so shutting down one and perating the remaining at high power (more effiecient) would seem doable for increased range. Also the smugglers have , I would imagine, a somewhat cavalier regard for rules, getting caught for flying on one, would pale into insignificance compared to a ton of coke in the back.