Will be as simple as doing a back-up from the 7100 then restoring it to the 8100? Or would that make too much sense?

Yes, assuming that there are no major changes to the handheld OS. I wouldn't expect any since it is a only a point release (v4.1 vs v4.2).

When I moved from my 7100t to the 8700g, I first made sure I was synced well with Outlook, and backed up using the Desktop Manager of the BB. I had no problems with moving all my address book, etc.