Of course it will shorten it's lifespan, but that said as long as the overvolt isn't too extreme it can be worth it as by the time the card breaks it will probably be obsolete anyway. Watch out though, overvolting can have a catastrophic effect on the heat produced, so keep it under control with EVGA precision fan control.
Remember, overvolting is useful to stop crashes from high clock speeds. It allows for higher stable clock speeds. Overvolting itself gives no performance advantage.
My GTX560TI is at 1050MV 950 Core 4400 Mem
That said no card is the same, some can push further without crashes, others can't even reach 950 MHZ even if it is the same type of card. Just try different settings, and test the heat and crashes using FurMark If it crashes, turn up the voltage or turn down the clock until it is stable. If it goes above 85 degrees in Furmark, it's too hot, so you need to turn the voltage down and clock down. Remember, to keep the very high clocks you need a higher voltage otherwise it will crash. Try different combinations, but I wouldn't ever go above 1068MV with the GTX560TI.
Extreme run on 3d mark vantage on New Calico with Asus DCII GTX560TI with 1680x1050
830 Mhz core
4008 Mhz mem (2004 - 1002)
max temp 66 degrees = 32.12 fps
950 Mhz core
4400 Mhz mem
Max temp 81 degrees = 37.30 fps
Without overvolting my max is
905 Mhz core
4250 Mhz mem
Max temp 72 degrees = 35.64 All fps are the average of 3 runs.
So yes, a performance improvement of 5 fps, in my opinion well worth it.
Running anything past it's stock speeds will damage it over time, especially overvolting, so just keep that in mind and don't go too crazy :)
Personal experiences with your card.