Why would Borgin and Burke be working against Voldemort? They own a shop devoted to dark objects. They would be fascinated with a horcrux, not repulsed.
Moreover, they are both alive, although RAB seemed quite certain he would soon be dead.
And neither of them have made any effort to join the Order, or in any way fight against the Death Eaters, which seems odd for someone who was willing to risk death to steal the horcrux in the first place.
The real locket Horcrux is almost certainly the locket that was found in the drawing room at 12 Grimmauld Place and that Kreacher stole out of the trash. If Borgin or Burke was RAB, how would the locket have ended up in the Black family home?
What Sirius says about his brother has to be understood through Sirius' bias. Sirius is angry at his brother for being a DE. He has the typical older brother contempt for a younger brother.
Besides which, we know that Burke's first name is Caractacus.
I don't understand the logic behind "it's too easy" as an argument. JKR does make things easy. Remember the Mark Evans fiasco? A passing reference was made to a boy named Mark Evans in OoTP. All over the net, speculation went crazy - ooh, the name Evans means he had to be related to Lily and this is Harry's long lost cousin. Nope. Just a coincidence, even though fans swore JKR never has coincidences.
And Dumbledore is dead? That's the simplest, most obvious conclusion at the end of HBP. Entire websites sprung up devoted to the idea that Dumbledore could not be dead, with some elaborate and very clever explanations of how he could have faked his death. But JKR came out and said, plain as day "Dumbledore is dead."
Ron and Hermione - the obvious romantic coupling, and per JKR - that's the way it is. Harry's mysterious power referenced at the end of OoTP. Oh it couldn't be love, that's too obvious, that's too easy - but nope! Dumbledore confirmed in HBP that it's love, "just love."
I will bet you a $25 gift card to the Disney Store that RAB is Regulus.
We don't know WHERE Burke is, Borgen is the only one mentioned as being in the shop any time they have been there. Also, just because they sell dark objects doesn't mean they are evil, Borgen certainly doesn't like Malfoy one bit.
The too easy comment is based on everything JKR has ever 'hinted' at in upcoming books. Most people think they are easy answers and they turn out to be something totally different. RAB is not Regulus Black, it is too obvious for her writing style.
Where in any of the books does it give Burke's first name???
The locket horcrux could very well be that locket and it could have been given to the Blacks, who were NOT death eaters other then Regulus, for safe keeping/hiding by Burke, easily.
One thing you have to remember and it was said in the books, being a bad witch/wizard doesn't automatically make you a death eater so just because someone was in Slytherin doesn't mean they are death eaters.