I am working with a Fibre glass Guy who will be manufacturing a smooth/paintable rear fascia to match the stock shape, but incorporate the diffusers. I will let you guys know when he has it ready, I believe he is a supporter of the Solstice site, and has done work for a few guys local to me.
Here is something custom he did for Spiky:
New!! SOLO Performance CSD Exhaust - Pontiac Solstice Forum
To me this looks better than the carbon fibre, UNLESS of course you make othr parts carbon fibre, than at least it will tie in. But if this is the only place you have carbon fibre, I think it will look out of place!