I like the idea of sanding it off, the remaining edge should supply good support for the H/S and help prevent crushing the core should the H/S get tightened unevenly. In theory you should cut through the face of the IHS, and still have the solder on the die. Did you measure the thickness before starting ? It would be interesting to know the actual thickness, or if someone has removed the IHS and has access to a micrometer to measure please post. Don't for get to shim the H/S to make up for the lost material, if you knew how much you took off, you could get shims of the equivelent thickness and remove the guess work.