I'm seem to recall hearing that there is real benefit in offering the courses because it DOES reduce the rate or re-offending.... otherwise, there would be little point in offering the courses, and all speeders would simply be fined/ receive points and be done with it.
In which case you'd think, from the general population of speeders, those who had attended the course were a lower insurance risk than other speeders, and potentially a lower risk than the rest of the driving population. It is, afterall, quite a good general driver-awareness course, which most drivers would probably benefit from.
I tend to think this is simply a case insurance companies finding an excuse to put up premiums, rather than any real risk-based research.