[spikels]

Phantomjs handles everything you mention (status codes on large numbers of resources, ajax, deferred loading monitoring and HAR output) with the possible exception of websockets - I have not tried and very little documentation today but it should work. The big limitation is this is WebKit-only right now.

For example: here's the wiki on network monitoring including HAR: https://github.com/ariya/phantomjs/wiki/Network-Monitoring

The API seems pretty clean to me but I guess that is a matter of opinion.