I am running at 780Mhz with a fan speed of 75% and I am pulling 75-77C on loads. Getting 100C with an 850Mhz OC doesn't sound abnormal to me with stock cooling (keep in mind room temps and case circulation play a factor as well).
What are your stock setting temperatures? That will be an indication if it will work for you. I changed mine and I only got a 2C temp drop. Nothing worth the effort but if your sink is somehow installed poorly from the manufacturers, then there is a chance it could work.
I had a 4870 that was running really hot and I was able to drop the temps over 20C with new thermal paste so it is definitely possible but only if you have a bad heat sink install out of the box.