Anything less than the 70 series is underpowered. I doubt seriously if there are any efficiency gains to be made by shutting one or more down in cruise on any -8 version. I do know that this has occured on several occasions, but for entirely different reasons. Some engine component times were reaching their limits and the scheduled trip could not be completed without exceeding them, so the engine was secured for a fair amount of the cruise portion. These days, I'd just refuse the trip, but heck, there was a war on!