Does anyone know about batteries?

I've been looking at getting an extended battery for my GS2, I was looking on Ebay and there seem to be a tonne of batteries much bigger capacaty than the stock battery that are the same size, so therefore do not require a different/thicker back cover.

And by much bigger I mean 2500 mAh vs the stock 1650 mAh...

Can these be legit? I was looking at this one, apparently 2500mAh, and it says it has "Comparable standby and talk time". So do these actually perform any better than the stock battery? Does anyone have any experience with things like this? Can someone help out a confused fellow Verge reader?