gtx 650 overclock

The 650 ti has twice as many shaders as the stock gtx 650. I do not think you can get a clock rate fast enough to catch up(Not that I have tried).

Chances are you could get close with a 1300-1400 + clock(and some good clocks on the memory), but it would depend on the game played for sure.
 
If you have no experience overclocking you should not just drag sliders and hope for the best.

All cards clock differently, some may even need hardware mods and aftermarket cooling to get to the place you want to be.

As the above users says, close is still 15% give or take worse. Simply put one card has HALF the number of shaders. You may even burn a GTX 650 power system trying to clock to far
 
higher clocks perform better(and take more power and make more heat), you have NO guarantee that the card will get + 300. it may crash at +50 or may get to +350.

Overclocking is a game of slowly increasing the clocks and testing then increasing then testing and so on.

Tools like MSI afterburner are commonly used, but gigabyte may have a tool for the cards they sell as well.