So I'm interested in looking into questions like:
What point difference does 1 rank difference between players tend to lead to? Does playing black or white have any influence on who wins these days? Do longer games tend to benefit, or hurt a lower ranked player when playing against a higher ranked player? How fast Go players tend to advance through the ranks, and what clubs and countries tend to have the highest growth rates.

Anyone know where this data might be freely available in a format that makes it easy to use?

Thanks so much!

For pros or amateurs? Do you know about the European Go Database http://www.europeangodatabase.eu/EGD/ ? That has a record of the result of essentially all European rated tournament games for the last 20 years or so (though just who won, not the score). If you contact the admin (Aldo) he may be able to give you the data in a more useful DB dump format.