Well you can easily find out what you've gained from your overclock by wathcing your fps when you're gaming compared to the normal settings. Your card is already overclocked from the manufacturer so I wouldn't expect you to be able to go much higher without increasing the voltage also.
Personally I never overclock my card until I notice a drop in fps when I'm gaming. If I can't tell the difference there's no need to do any changes. Only when the day comes that I feel my card is lacking speed when gaming, then I will start overclocking. But that's just me. Some overclock their card the day they receive it.