The interview on TrekMovie says that they're using a CG Enterprise for shots they don't find, and they're using Tobias Richter's model, better known as The Light Works. I've always been impressed with his models for years, he now does the effects for New Voyages/Phase II. I love when they don't start from scratch when there are perfectly good fan works out there. Interesting that they didn't use the model created by John Knoll for Generations (and used in DS9 as various Galaxy-class ships) OR the one Gabriel Köerner (another ascended fan) made for the last episode of Enterprise.