Even though they're suspected GPL offenders (opinions differ) I still have to put in my word for mikrotik. These guys know how to build wifi in rural areas with plenty of subscribers, stable hardware and good software at low cost. Even their cheaper products are very well up to the task and can be expanded upon with different wireless-transmitters and antennas. If that is not enough you can always look at their more "enterprise:ish" products. I've only good things to say about them, and we used their products for well over 5 years when we still ran a WISP.

Here, go ahead:

http://stats.autonomica.se/mrtg/sums_max/all_month_sum.png

This is the monthly summarized statistics for netnod. Does indeed look like a big drop at April 1st and pretty stable usage before.

How does your upstream contracts work? You could try and see if you can buy traffic per 95th percentile with a commit instead if you need the burst capacity . Then throttle the worst offenders if you notice your 95th percentile going over your commit.

Howtoforge.org has fulfilled that role for a long time which Linux.com aims to do now.