Laptop upgrade - 3rd Party Battery/Heatsink Replacement + Graphics
So, my laptop battery is once again coming to it's end-of-life, and this time Apple won't replace it for free (they did it last time as it was under Apple Care and it was Christmas - the guy was being nice). I'm wondering if there are 3rd party batteries, that aren't Apple Manufactured, that actually are larger in capacity but identical in size. I mean, batteries HAVE evolved in capacity in the last 5 years and it stands to reason that the same sized battery would now carry larger charge.
While I'm at it, are there also now more efficient heatsinks/fans I could throw in?
Final question - would anyone here be able to explain to me why exactly it isn't possible to take out my DVD drive and replace it with a laptop GPU? I'm aware that since I only have Intel Graphics, I can't well just remove it and replace it. But if it is possible to connect an external graphics card, why is it generally suggested that it's impossible to connect one internally?