Question: Is your memory rated at 1333 or 1600?
I've ran into this, where I can run my 1600 ram @ 1333 at 1.5v with no problems. That may be because it is rated at 1.65v but at 1600. Either way, I exchanged it for some firstname.lastname@example.org so I do not have to worry about the issue any more.
Edit: If it passes memtest and a few hours of prime95 I would think you can take that as a good sign. If it works with the 1.5v, you will not be doing any harm by running it that way.