First, monitor your temps. Sometimes the CPU works hot and it clocks down to prevent overheat. A better cooling could improve CPU performance by making it sustain higher clocks.
Second, if temps aren't the problem and it works at its rated clocks, try OCing it. You will also need a better cooling solution and maybe a better PSU.
BTW, what PSU do you own? Sometimes a bad quality PSU can hold a CPU or GPU performance.