Greg,
I have the first foam core Shrew, not the first carbon backed Shrew. Gregg Coffey has been putting carbon on the backs of bows for some time now.
I have several of Bob Morrison's bows with foam core limbs and some of those have carbon backing too. Both Bob and Gregg can put exotic wood veneers on top of the carbon backing, but then they are unable to apply a layer of clear fiberglass over the veneers. Instead, the veneers have the same sprayed on finish that is on the wood risers. This may not be quite as protective as the fiberglass, but it works just fine for me, and the limbs look awesome with the fancy veneers.
The Classic Hunter I will get this July will have foam cores, carbon backing and exotic hardwood veneers. The Lil Favorite will have foam cores, hardwood veneers and clear fiberglass over the veneers. Because the recurve limbs on the Lil Favorite are thinner from the bellies to the back than the longbow limbs, carbon backing actually hurts performance rather than enhancing it, and the smoothness of the limbs suffers too. So carbon backing is not a good option on the recurve limbs until you get into higher poundage bows where the limbs need to be thicker anyway.
Shrews are not known to be really fast bows, but with foam cores and/or carbon backing, they will be very competitive with many of the faster bows on the market.
Allan