Probably Gustaf is better suited to answer your question about ns_http -- since I haven't had a chance to use it (much) myself. His mentioning of direct usage of C-level API and better integration into AOLserver threading are probably compelling reasons to investigate.
We use a combination of Tcl http and proxies curls ourselves, although the latter is reserved for very specific tasks such as file uploads. (In fact, probably these task would be better managed through a queueing system that would run the uploads entirely outside of AOLserver.
