You sure know when you're lugging an 3-cylinder!
You definitely get an engine note & vibration that isn't nice (as well as not being able to increase or maintain engine RPM when needed).
I'd say the factory shift recommendations aren't in "hypermiling" territory.
I haven't met a 5-speed that won't cruise (on the level) in top gear at 25-30 mph. It probably won't climb or accelerate from that speed without lugging, though -- that's what downshifting is for.