Interestingly though, there is an argument that even ISPs don't have the right data, and that this is a reason - or at least an excuse - for throttling. See http://www.bobbriscoe.net/presents/1005ftw/1005ftw-briscoe.p...
There is an ietf working group (ConEx) on exposing the kind of information necessary, with the idea that this would provide the right kind of incentives for ISPs not to throttle. I'm not sure that ISPs wouldn't anyway have the incentive to be a gatekeeper, but it would be a good idea to remove any possible excuse.
Or, you could refer to the product you're interested in buying, as they can provide information about how well different internet providers can service their product:
