This is a most informative article detailing some enlightening research by academics such as cognitive neuroscientist Marlene Behrmann, involving face processing by regular people and by different types of prosopagnosics. It turns out there are important differences between different types of prosopagnosics. An important theory in the science of face processing, the “face space model”, is explained in this article, and it also explains who the heck Dan and Jim are.
Although synaesthesia is not mentioned in this article, at the end of the article there is a great big hint as to why synaesthesia might be associated with superior face processing abilities (super-recognizers or high ability), and why an under-connected brain might be associated with developmental prosopagnosia (congenital face-blindness).
Zimmer, Carl The brain: seeing the person behind the face. Discover. January-February 2011. Online January 19th 2011.