It's just like most things in electronics, you take your chances with what you get. One refurb might o/c better than a brand new card or vice versa. It all depends on the magic smoke and electron fairies in the board and your ability to lure them out.
Unless the card was massively hooped when it was RMA'd then the card should be every bit as good as a brand new one. And even if there was problems, they are typically sent back to the manufacturer for repair. They fix it and then turn around and sell it. Albeit at a lower price as it is no longer "brand new".
On the other hand you are probably saving some money and it depends what you want more: megahertz or moolah.