Well, there was always a hint that despite their being different, deep down, Elena always has something in common with Katherine.  The hallucination Elena had of Katherine pretty much forshadowed she turning more like her.

I thought this seen ready made it obvious, when Damon told Bonnie that Elena already is like him. http://www.youtube.com/watch?v=7IA39IzI2ow

Your thesis in your assumption is that Damon is like Katherine. First that is farthest from the truth. Damon may show the world most of the time he only cares about himself. The truth is he cares deeply for those close to him. There is many evidence of this, from Rose, Stefan, Elena and even to looking to free Katherine when he thought she was imprison. To the simple fact he saved Caroline life with his own blood when she had been in the nasty car wreck with Matt and Tyler. Katherine is only about Katherine, everything she does is for her benefit or not at all.

Yes Elena has changed, she is a vampire now. But her caring for others and putting them first is still very much evident. The reference you point to merely points out that she is more like Damon in how she handles being a vampire in regards to moving amongst human society that she truly is no longer apart of with being a vampire. That if she does kill someone inadvertently. She will not be shattered, that she is made of stronger psychic. That she won't go into a shell or turn into the ripper. That is what was meant by his statement to Bonnie.

This isn't even a possibility.

Elena's behavior change can be responsible for her sire bond. But yeah, I guess she's turning into Katherine but not in an evil way. There is no way if she's being selfish like Katherine, her heightened guilt will practically kill her.