Obviously it would be less expensive to just buy an E8000 display, but if you have an E7000, it is easy to use a Garmin 830 or 1030 as a display, and it will collect all kinds of data for you as well. You just leave the E7000 on the bars and use it when you don't have your Garmin.
If you have an E8000, you have to spend $100 on the Di2 bluetooth dongle, and remove the E8000 to use a Garmin. Dumb.