Yes, the 780M performs better than the 870M (http
/www.notebookcheck.net/NVIDIA-GeForce-GTX-780M.88993.0.html), it's ranked higher than the 870M and has a higher clock speed. However, the 800M series GPU are designed to be more battery efficient than the 700M series so there's some benefit to the newer cards.
As for the 765M SLI, the 870M is clocked at a higher speed, it's more battery efficient, and it produces less heat. SLI GPU's generally produce more heat than single GPU's. So if I had a choice, I'd opt for the 870M, it's an all-around better GPU.