I agree that the 8-Series coupe will fail, but mainly if its styling doesn't match up to the LC500 and S-Coupe. Both of those coupes are seen as incredibly beautiful designs that strayed from previous iterations significantly (CL) or any previous design from the brand (LC). The 8 Series concept looked like it had some promise, but the camo'd test mule deflated those promises quickly.
Where I don't necessarily agree is the impact on the 6GC on the 8Series coupe. Of course it depends on the performance and total package of the 8Series and how it creates separation with the 6GC. Just like the CLS and S-Coupe have enough separation that there is little overlap. Same with the S-Coupe and the upcoming AMG-GT 4-door.
Source : http://www.autospies.com/news/Audi-Design-Boss-Wants-To-Produce-An-Answer-To-BMW-s-8-Series-95198/