The campaign has been very successful in getting high ranking for several targetted terms. However, I am seeing zero impact on our site traffic or sales.
I am beginning to wonder if Google's algorithms are so efficient that all of this extra SEO work is to no avail.
All you're setting out to do with SEO work is impove your rank though, if that's happening but you're not getting traffic then it's possible you've been targeting the wrong keywords.
Can you see your clickthrough rate in webmaster tools for these new keywords? Are they exceptionally low? Perhaps your meta description isn't appealing and you're ranking are all for nothing.
I would work on bringing those people who see the results to the site by testing the meta descriptions, supplementing the link building with some social stuff to distribute your new content and a bit of conversion rate optimisation to capture those who do click through.
I would say you're never done with SEO though, just need to change the focus of your efforts to capture visitors who are going to make you money. If you stand still somebody will eventually beat you