At least to my ears, the carbon cellos are just plain bad. Hear one there[...] The carbon fibre cellos I've heard actually sound pretty good.
5SRDj9xGAoM at 7s
I admit two advantages: the instrument responds very quickly and it doesn't mix the notes up. I hate everything else: no depth, sounds like a piece of plastic. It's about the same difference (though smaller!) as between a guitar and a banjo.
But maybe a sandwich of graphite-balsa-graphite might be decent. The sandwich is potentially lighter than spruce at identical flexural resonance frequencies. The trial(s) would need some physicist to "predict" the thicknesses (in two fibre directions!) that reproduce the resonances of a wooden instrument.