I've seen steel analyst use a damping ratio of 2%, so it doesn't suprise me that 2% would be conservative. I've seen thick balsa cored (3 to 6 inch) composites with as high as 6%. If I had to take a guess, assuming a carbon fiber of modulus 30e6 in an epoxy resin, I would say 3% would still be conservative. 4% may be close...but that's an educated guess (with a strong emphasis on GUESS.
Garland E. Borowski, PE