Planning poker focuses the team discussion on what matters, and allows quick movement on from what doesn't. It gets the entire team on the same page about what the stories are, and in general agreement about their effort cost.
Picture: team around the table, story one, everyone shows the same number. Great, put it in the bag and move on. No more wasted time on that one.
Next story, 5 different estimates. Okay, that's weird - discuss.
Oh, you mean x? I really thought this
was dealing with y. Well, I don't
know, maybe it is y. What do you
think? Okay it is. What about x? We
need another story for x, then. Okay,
break it up - this is y, that is x.
Estimate y, similar numbers. Bag it. Estimate x, similar numbers. Bag it.
So, where you might have had one guy estimating a story incorrectly due to a misunderstanding of what it actually dealt with, now you have the whole team with a common understanding of what the story is, and agreement on what it will cost in effort. The important part is that you've headed off potential downstream troubles related to poor understanding of the features at the pass. Consequently, your estimates are probably much better.
However, no, I do not know of any data to substantiate any of this.