A GPU's performance should not degrade over time. Like said above I would find out why your card is not performing as well as it used to before you consider overcloking.
As for the safety of overclocking, I have overclocked every GPU and CPU I have ever owned and never had an issue, my old gtx 760 saw heavy use with a heavy overclock its entire life, I used it for several years and it still works to this day. Heat is what kills electronics not overclocking and with your msi twin frozr gpu heat should not be an issue assuming its not caked with dust. You will more than likely upgrade long before the GPU dies, with or without any overclock something like msi afterburner and the stock bios for your card will allow.