And here's my update - the battery won't physically fit. And the power pads do not match. I've modified it to fit the Xperia V and am currently testing its durability. Here's how you can make it fit the phone:
There's a plastic frame all over the battery. You can brake it at any place, but it's best to brake it on the left hand side as you'll need to remove the left part of the frame. The battery is wider than the B800 but is just a bit shorter than it. So you need to make some space.
If you remove the left part of the frame, the battery will fit, but the power pads won't match the phone's connectors. Therefore remove the top part of the frame (easily done on my battery) and you'll see the two metal connectors for the + and the -. There's also a small board that probably makes sure the voltage is ok. You can bend the connectors so that you'll move the pads a little bit to the right. Now the pads should fit the phone's connectors. For stability it's good to place back the top part of the frame, cutting one of the edges, cause it's a bit wider. The battery should also have the bottom part of the frame so that the pads will be well pressed against the phone's pins.
You can use double adhesive tape to secure the frame if something's not right. I can't make pictures right now, but if someone needs some, just let me know and I'll think of something
Last thing to add is the bottom phone's cover - because the battery is a few mm wider, it closes a bit harder. You can't see it if you look at the phone though.