Roughly half, but since the shroud does not actually direct that airflow at all towards the rear I doubt half of it is actually doing what they say. In any case, in cards like that you actually don't want the airflow out the back because if you didn't your RAM and VRM wouldn't get any flow. On reference blowers the plate covering all the components on your card gets proper airflow because all air is directed towards the back passing over that plate and through the GPU heatsink, then being expelled out the back. On cards like this the airflow is being pushed down through a much larger fin array to conduct better cooling on the GPU itself, while the VRM and RAM chips get left over airflow after traveling through the fin array on the heatsink. If any of it went out the back they would be reducing the cooling capacity of the rest of their card.
This is why reference blowers are a much better and efficient design and why I support them so much. Yes, there is a smaller heatsink for the GPU itself but the rest of the boards circuitry doesn't get left out AND all the air flows out the back. Oh, and it's a lot easier for enthusiasts to get water blocks for those boards.
To make perfect example, my Galaxy 580 has a ridiculous triple slot Accelero cooler on the GPU itself but tiny passive heatsinks on the VRM and RAM that are tied to a large heat plate on the back of the card which depends on case airflow to cool. They get so hot they put a warning on the heat plate to not touch the surface during use. There is good reason for this, because it'll literally burn you. The difference in VRM temps between the two cards is amazing.
"Resolution is just a number." #Ubisoft Origin/Steam
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