The 8" upgrade is going from the standard 5" heat panel to an 8" heat panel. This options includes the e-upgrade in its price. This option is best for racks in rooms with a temp of 74 and below.
The e-upgrade is an insulation added to the bottom of each heat panel. This ensures all the heat is going upward toward the bottom of the tub it is heating, and not lost out of the bottom of the panel. This makes the panels much more efficient.