I took mines off when i was painting the wing and i felt that the air flow actually increased without it. Dont think im going to bother putting it back on.
I have two actually. One for the heater which by my opinion increases the cabin noise level. I only use in the winter time if at all. The second one is connected directly to the airfilter - with a flexible hose ..
Defender TD5.
The scoop on my heater does make a marginal difference to the volume of air coming through the heater, but only over 60mph...and that may just be the other drafts around the soft top