I'm sure I read somewhere that only about 60-70% of people with PhD's end up in the field they studied! If you're studying something so narrow the chance of a job with the same topics is rare, so I guess most people end up with jobs that are 'somewhat relevant'.
Following threads on this forum of people who have recently gained their PhD's leads me to believe that industry jobs often believe you are over qualified if you have a PhD, I guess it depends on your field as all fields would be different. Snoop around on the websites of companies in your field and see if they have staff profiles, it may show you if the staff have PhD's which may let you know if it is worth you spending 3-4 years on a PhD?
Also how do you 'fast-track' to a PhD? I haven't heard that term before?