Part of my mining hedge is resale value of the cards. Removing the shrouds would kill this.
The shrouds can usually be removed without damaging the cards or the shroud.
I have a 6950 that just runs HOT, last night I experimented with removing the shroud and the stock fans, it was literally a 4-screw affair to loose the shroud from the heatsink.
I slapped on a couple of 110CFM 120mm scythe fans and they were able to keep the card fairly cool, but didn't perform any better than the stock fans with the shroud in place. It was ugly as sin, and I was getting about 88C either way. I've gone back to the shroud + stock fans just because it's cleaner and less prone to failure.