http
/www.notebookcheck.net/AMD-Radeon-HD-8850M.87118.0.html
"Equipped with GDDR5 memory, the HD 8850M competes with the Nvidia GTX 660M. The DDR3 version of the HD 8850M is expected to be a bit slower, although it could offer higher clock rates."
If you look at the ranked column on the right of that page, you can see the 8850M sits just below the 660M. They're pretty close in performance, and I put the 660M link above because it has benchmarks. You can expect similar results with the 8850M.
Hope this helps