Have you ever seen what happens when one of those batteries has a catastrophic failure? Might want to look at some YouTube videos before you put one of those next to your head.
It's not like Apple's batteries are perfect. There are numerous cases where 3GS's battery expand and explode. iPod nano needed a worldwide exchange because its battery expands and explode.
I would defffff not get that one, It is more expensive and uglier than other ones that look stock and they are lying about the capacity.
The battery says 3.7v (like a normal li-po) and 5.25 watt-hours so 5.25/3.7=1420mah which is the same as the stock iPhone. I have some from amazon that were around 5 bucks and look like a factory replacement.
With lipo batteries, if they start to puff at all you need to get away from them and replace them. I know from rc hobby stuff. I had a couple $500 boats nuclear out on the water.