Seidio seems to be in the habit of overrating their batteries by about least 20%. Frankly, I don't like buying from companies that have those kind of dishonest practices. Not that all companies are honest, but if a product is advertised to me as a certain spec, I expect it to be that spec or at least a reasonable equivalent.
Seidio's "slim extended" batteries are a prime example. It's a damn battery. You can't give it substantially better life at the same size without a new material in the battery. So all of their ~1700+ mAh batteries really test out at around 1500 mAh - the same as the stock batteries they replace. Cognitive dissonance does the rest. Or, those that own phones like the Incredible, that actually have space under the battery cover, replace their 1300 mAh with 1500 mAh, and never know that they're still 250 mAh short of the advertised capacity.
Anyway. Just sayin'. The 2800 mAh battery, if history is any kind of indicator, will probably clock in at ~2250, or around a 50% increase over stock, not double the battery life.