The promised patch never materialized.
One point, though, is that ManifoldCF is not single-threaded in any case, so you'd be unlikely to gain much in performance by going "multithread" on an already multi-threaded connector implementation. The current connector can maintain and use as many connections to Solr as you tell it. Memory buffering on the client side also is not a good idea because it violates the basic ManifoldCF principle that you can safely shut down and restart ManifoldCF at any time without loss.
Solr also suffers from lack of a "guaranteed delivery" metaphor, which I've talked to the Solr team about in the past. The Solr commit model currently does not work this way but ManifoldCF really requires it, because without it there is no way to properly implement an incremental crawler. This would mean a significant new Solr feature.