The standard reference design is easily capable of overclocking to the same speed as the stock OC editions, BUT if you want a higher overclock than stock, keep in mind the OC editions have upgraded coolers so overall they tend to be capable of higher overclocks. If you're looking for the highest possible overclock go with the OC edition so you have the better cooler.
So can a Sapphire HD 7870 overclock as good as a Sapphire HD 7870 OC Edition?
Just wondering because the OC version costs 15€ more
Here is the difference between non-OC and OC editions:
OC editions: These are factory overclocked cards and are guaranteed (by warranty) to be overclocked and stable at the rated speed settings. You can try to go above the rated speed however, this is not guaranteed to work, it may void your warranty and it may damage the card.
non-OC editions: The ordinary non-overclocked editions of the card are only guaranteed to operate at stock settings. This does not mean that you cannot try to overclock the stock editions of the card, however, you most likely cannot RMA the card back to Sapphire claiming that it does not overclock, since Sapphire never claimed the card was overclocked.
Keep in mind that for some manufacturers, damaging a card due to operating it above factory settings may void the warranty (regardless if the card is an OC or non-OC edition). This is similar to the way that you immediately void the warranty if you overclock an Intel i7 2600K cpu. Even though the CPU has the K suffix, and it supports overclocking, intel states in their 3-year warranty specification that the warranty becomes void if the CPU is overclocked. Silly I know, however it is true.