That's an interesting thought. If a goodly volume of air is being slung off the blade tips, it would seem having the tips outside the shroud would allow more 'fresh' air to be pulled through the shroud. If the tips are inside the shroud it is causing the slung off air to be recycled causing less 'fresh' air to be pulled through the shroud....it would seem. Perhaps that's why many fans have the tips angled back. To enable the fan can be installed deeper into the shroud for safety, but still are able to route the air slung off the fan tips towards the outside of the shroud.....?