First & foremost, the shroud IS important. If the ‘chunk’ that is missing is large, a lot of air will recirculate from in front of the blades out the rear & back in again, thus less air going *through* the core. This will be most apparent during low speed driving, when the ram air effect is minimal. Centering is not as important, but may have some effect due to the vortices created by the ends of the blades.