I agree with you when you say that there is more to car than its design Nick, But that's pretty much all I agree with you on from your post.
If it was me trying to create an engineering masterpiece, I'd want to go the whole way, Not do it halfheartedly, Like i feel Hennessey have done here. If hennessey had created their own design then I'd probably love this car quite a lot, but because they took the easier route and used someone else's I rather dislike it, Sure It's going to be very very fast, But it's not 100% "unique", If i was hennessey's CEO, I'd never feel 100% proud of my product, as it isn't all mine, I used parts from someone else's car. If lamborghini based it's new Murcielago on a Ferrari 599, tweaked it and added a new engine, would you be happy to call it a lamborghini - I know I certainly wouldn't.
Design plays a really big role in how I judge a car, It is important. I love the small details you find on some cars, I love the curves, the edges, the bumps you find on a car, it lets me know how much passion went into the design of a car, which in turn lets me know how much passion went into the rest of the car, the engine, it's performance, the interior etc. Hennessey put no passion into it's design, what does that say about the car as a whole?
Of course this is just my opinion, and everyone is entitled to their own, I'm just not a fan of people using other people's designs (or badge engineering).
Sure this car is going to be amazing, But Hennessey could of done better, and really should of.