Vita Slim Battery Mods - General & Specific Questions

After some searching I found this mod:


Blog post with more pictures:
http://www.ycptech.com/2014/12/ps-vita-play-time-marathon-5520mah-extended-battery/

And I'm wondering if anyone else has done anything like it and what is required as I assume there is cutting involved similar to the fat vita mod. I don't currently have a slim to open up otherwise I would check on my own.

I've seen concerns about using a smaller battery and having it being overcharged since they're obviously not the same capacity. I also see that both batteries are wired to the same points, is that the proper way of doing it?

I'm also curious if there are any higher capacity batteries that will fit in place of either the original or secondary battery.