Hello,
I'd try to use postgis ( http://postgis.refractions.net ), that allows
you to geolocalize your data. We use such an extention here for the
same kind of data you use. It will lighten up table size, and queries.
Otherwise, partitioning and cluster are good things to do. I would
increase default_statistics_target too. Up to 400, maybe ?
I don't know how/what for you analyse data, here we've clustered on time.
HTH.
Laurent