Comment 223 for bug 1040557

@Fabio: if you completely remove Win8, you will likely lose the ability to upgrade your machine's firmware. The firmware can be upgraded only with Samsung Updater - a Windows application.

If Win8 does not start from your newly installed Grub, you are probably hitting the known bug:
https://bugs.launchpad.net/ubuntu/+source/grub2/+bug/1024383
The solution suggested in post 18 on that bug's page worked for me. However, there was another problem with incorrect timeout setting; see paragraph 14 in https://bugs.launchpad.net/ubuntu-cdimage/+bug/1040557/comments/114/.

Apparently, there are other ways to get Win8 Grub entry working. See post #189 above.