Well, the equivalently strong thing seems to be mostly something that's come to prominence in the last book. We see that when he was still in Hogwarts, he was inventing new spells, and correcting the accepted knowledge with his own observations. This sounds suspiciously like Dumbledore at his OWL (or NEWT, can't remember) examinations, where the examiner said that he had done things that she had never even seen before. Likewise, Voldemort was atypically strong for his age. I think it's somewhat clear that we're being shown that Snape is somewhere in their vicinity, if not at the top along with them.
Him being a master of Occlumency means that it would not be unusual for him to be able to hide his true strength from Voldemort. Voldemort has never tried to intimidate Snape, as far as we know, and may well have been forced to trust him via circumstance... well, as much as Voldemort trusted anyone.
I'm tentatively in the "Evil" Snape camp now, after having reread the chapter of Dumbledore's death again, though perhaps he's only "evil" by circumstance (I also find the idea that Snape was in love with Lily to be quite likely, so him feeling spurned could have led to his turn to the Dark Lord). Snape made an Unbreakable Vow to help Malfoy, so there was no way that he could not have killed Dumbledore. But on the other hand, he doesn't show a sign of hesitation. He doesn't hinder or obstruct the other Death Eaters in anything, except in that he spares Harry, and that's my pet theory as to what Dumbledore was pleading about--he was appealing to Snape to leave Harry alive so that he could face Voldemort. Note that Malfoy notices immediately once he gets on the tower that there are two brooms, and connects the dots; it would not be surprising if Snape managed to do so as well, so I'm guessing that Snape knew Harry was around. Snape, of course, will have heard of the prophecy, and knows that Harry facing Voldemort is perhaps the only way the conflict can be resolved. Therefore, it was in his self-interest not to kill or seriously injure Harry. Therefore, just about everything in that chapter can be explained by him being either "evil", or entirely self-motivated (another kind of "evil", in any case).
While it's possible that he and Dumbledore worked it all out beforehand, I do not think that it is likely. Dumbledore taught Harry very little, all things considered, and is leaving him woefully underprepared to face Voldemort. He fights for life, as we can see throughout the chapter, him managing to stay on his feet despite everything, negotiating calmly with Malfoy. Dumbledore is surprisingly accepting of death, but then, Dumbledore, in Book One, said that death is merely the next great journey (paraphrasing to some extent). He's never been particularly afraid of death, and he wasn't in the chapter, but I do not know that this means he is ready to die.
