how often do you figure you're going to be changing that sprocket? If you're riding on the street, probably not that often; once you've figured out your preferred gearing, that is.
There are other options for multiple gears on your fixie.
1. fix-fix flip flop hubs.
2. level bolt-on-cog hub.
3. miche adapter on any other (non-campy?) track hub (double check this).
4. eno 2-speed fixed cog ($$$).

The thing with the Miche group is that you're forced into a couple proprietary Miche components, most notably the BCD of the chainrings, and the weird splined cogs. Plus, the hubs have gotten some bad/mixed reviews on this forum.
But then again, $285 for the entire primato group including that sweet sweet seatpost is a pretty enticing prospect. I'm considering it for my next project. I just don't really think that the splined cog system is enough to base my decision on.