The cargo volume expands in step with the square of the radius of the hull. Thinner, faster ships cannot haul as much cargo.David Surdam's book on the blockade makes it clear that while the blockade could be evaded by specialized blockade runners, these specialized vessels had a low cargo capacity. Just the fact of the blockade cut cotton exports so much that the CSA could never have accumulated enough income from the exports to pay for any significant increase in imports. Plus, the blockade interrupted the Southern state's vital sea lanes with each other--a huge part of the prewar transportation infrastructure.
It's not just CSA to Europe that needs to be considered, but also one part of the CSA to another.