Another thing about 6.5 ROMs, I don't want to overwhelm you with having to learn a whole new setup but I think it will be easier to essentially start off with a clean slate. A custom ROM does in fact wipe out the original Samsung-modified version of Windows, and you end up with a new version of the OS with little to no added crap. It is an entirely new OS rather than a shell like SPB which coexists with what you already have. The 6.5 interface on the whole is much better, and comes with a new home screen that looks like this:
Since many parts of the OS are overhauled, you'll have less work adding third party programs to make up for the shortcomings of the built in functions and the phone will probably run faster.
Whenever I flash a new ROM, I run PIM Backup to My Storage which saves my important contacts, messages, appointments, speed dials etc and all of my other data is already there, so when I flash back, I just have to restore with PIM Backup and reinstall some of my programs.