The difference in clock speed is a bit more than 11% in base and boost clock. You are getting a good factory overclocked option. As boost clock depends on a lot on the cooling ability of the graphic card, it is easy to assume a non-reference card ( in this case- the AMP! Extreme) will do a better job at providing better performance.
Slow computer got you down? Maybe a hardware upgrade should be in your cards, if so this guide to the Fundamentals of Upgrading a PC is an invaluable read. PCSTATS Tips