[HighMPG]

Yea, so taper it more? That's what I was suggesting but what are your plans now? extension or different angle? The civic has a pretty nice rear end already and slope from the roof to rear glass to trunk.
I think you may have to build a kamma-back for the trunk which should help more because in my mind, that's where all the original turbulence is. When you made this style it shows you made the drag greater because the air leaving is trailing off a larger/higher edge, I just have these ideas since I have been looking at many different cars for good rear aero since it seems where most of the improvements go. Your car is already a kammaback up to the end of the trunk, I think kamma backs are left for the hatch backs or non fastback style cars. You could use Vortex Generators on the rear of the roof before the glass and tape up the gap between the trunk and glass to get the air to stick more.
Since I am getting a 95 Hatchback I would totally go Kamma back or tapering tail but for a coupe the trunk is the best spot to build it off of imo. you may see more improvements there but I would love to see a Civic Coupe Boat tail!