The reason my original mod didn't work was because of an installation foo.
On the reverse side of the card there's a x-shaped plate that squishes a rubber pad into the card and pulls the heatsink down onto the GPU. The plate is curved so that it has a concave side and a convex side, and I had put the concave side toward the card.
This is wrong!
You want to put the plate on the card with the concave side facing out.The reason it's curved like this is to create additional pressure on the nuts that hold the screws from the heatsink onto the card. When I had it on backward, it wasn't creating any resistance against the nuts holding on the heatsink, so the heat transfer was weak between the GPU and the heatsink.
The instructions could be updated to make it a little more obvious which direction the plate should face. Or I could just be a little less NUBISH.
Now my card idles at 55 degrees Celsius versus 65. I haven't had a chance to play any WoW, so I'm not sure what it will look like under load, but I'm sure it'll be better than the 78-80 degrees I was seeing before.