The max overclock is whatever it gets up to stable, all 980 ti's are different. 1450-1460mhz core and 4000mhz memory seems to be the high end for 980 ti's but some will go higher some wont make it that high. It's perfectly safe at any clock speed assuming temps are in check, the only thing pushing an overclock too far will do is cause instability and crashing or lock up's, if that occurs just restart and lower the clocks.
The nvidia BIOS wont let you go past 1.23v, which is fine most 980 ti's would handle a lot more than that if nvidia allowed the voltage to go higher. So if you are using msi afterburner or something just max out the voltage and power limit slider when overclocking.