I don't know if you can call it 'ducting', but I boxed in the area between the lower grille opening and the radiator of my Vibe. You can see it in
post #53 of my 'Vibe mods' thread. With the upper grille completely blocked, and the lower opening blocked roughly 90%, coolant temperature is never an issue. In fact, I think it's taking LONGER to warm up than before. I noticed this immediately after installing the radiator-bumper 'ducting', which was before winter hit hard and we still had comfortable ambient temps.
If your car has a similar gap between the grille and the radiator, I would highly recommend boxing it in or 'ducting' it. You can get away with a much smaller grille opening. Of course, keep an eye on the coolant temps, preferably with a Scangauge.