I am using my 950XL with just a Mozo back and a tempered glass screen protector. I am paranoid about my gear, so there is not a scratch on the phone, the Mozo or the screen protector. Then again, I don't have a scratch on my naked 920 from way back when either.
To answer your question though, the Mozo + tempered glass solution will not offer you complete protection in the event of a drop. This is because of the black plastic rim around the glass on the phone itself. It is part of the screen assembly and forms the upper corners of the device (when facing up). This plastic rim is left completely exposed even with the Mozo + tempered glass combo equipped. So if you drop it on one of the corners with the plastic rim, it will be the phone itself absorbing the impact - not your sacrificial Mozo back cover or tempered glass protector.
If you do not drop your phone, however, I think the Mozo + tempered glass combo will offer very adequate protection for daily use. Of course, the Mozo has the benefit of being less slippery and easier to grip than the stock back cover due to the flat sides.