Unfortunately, nothing is guaranteed when it comes to overclocking. There's a chance that if you buy a regular 570 it won't overclock a single MHz more on the core (although chances of that are very unlikely). Conversely, there's just as much of a chance that a vanilla card will overclock much higher than a Superclock could.
The way I see it, Superclock cards (and any factory overclocked cards for that matter) are meant for people who do not want to mess around with overclocking and would rather keep the cards as they came from the factory. Therefore, paying the extra $20 is a guarantee to have a card that is faster than the vanilla variant, and in the event it ever does break, EVGA will take it back and replace it with a similar card. However, if you're not averse to getting your hands "dirty", you can easily save the money and do it yourself as EVGA is pretty lax about overclocking anyway, so long as you're not doing some hard volt modding to the card.