I've noticed that WriteLineDocTask declares it does not support multi-threading, but taking a closer look I think this is really for no good reason. Most of the work is done by reading from the ContentSource and constructing the document. If those two are mult-threaded (and I think all ContentSources are), then we can synchronize only around writing the actual document to the line file.
While investigating that, I've noticed some 1.5 TODOs and some other minor improvements that can be made. If you've wanted to make some minor improvements to benchmark, let me know . I intend to include only minor and trivial ones.