I've bought a bunch of 3rd party batteries for my previous phones (Nexus One, Sensation), and have found that they're pretty much all crap and any perceived gains are simply psychological. Actual batteries mAh ratings are often significantly exaggerated on the product labeling, this can easily be determined by peeling off the label and looking for serial and model numbers printed on the metal battery casing. Googleing these numbers brings up product data sheets from manufacturers in China with the actual mAh rating.
I now only buy OEM batteries as I think it's the only way of ensuring your getting what you think you are. Think of it this way, if Samsung could pack more mAh into a specific size, they probably would. The size of a battery is limited by the battery technology, so if you calculate the mAh/gram density of the OEM battery I would use that as a guideline. 3rd party batteries advertising densities higher than that are probably exaggerating the actual capacity.