Were Carboniferous plants more efficient sequestering $\ce{CO2}$ than present plants?
Probably not.
The biological pathways to make lignin is the same in all plants so there is no reasons to believe they would be better. If anything modern trees as a whole may be better since they survive in  larger range of environments and very dense wooded trees exist. In addition without defenses against modern wood decomposers Carboniferous plants may have a hard time surviving in the modern world.