Odd indeed-- I don't consider it a pathology either, although I know some people with synaesthesia experience minor difficulties with basic math (numbers adding up to the wrong colors, for example, as in the excellent novel "A Mango Shaped Space") and some sensory overload problems (commander_d, for example, has tastes associated with colors, which can make art appreciation a bit hazardous).

It is true that many people with exceptional memory, including Luria's mneumonist, are synaesthetic. I believe the ability to associate things so heavily is a boon to memory rather than a detriment. Either that, or the neural differences that cause synaesthesia are in some way linked/related to those that increase memory.

Have you read "Born on a Blue Day"?