I really have little or no idea about how effective "High stakes assessment in support of policy" is going to be.
As things stand, I'd guess not very much, so long as the people in the 'educational system' - the 'policy makers' and the 'implementers of policy' and the other stakeholders don't get their basic thinking somewhat better focused than they seem to have at the moment.
I believe that the various sides to the argument all seem to be missing the fundamental issue:
What is needed is to enable the learning abilities that all *normal* students naturally possess. Evidence that we are NOT enabling those learning abilities is available from the fact that the great majority of students who pass through the school system come out fearing and/or loathing math. If I'm not mistaken, President Barack Obama himself - surely a person possessing exceptional intelligence - had recently confessed that he was poor at math during his schooldays. I believe things have not changed much since then.
Math education and other education - it is all quite largely ineffective. True, some students do manage to pick up useful knowledge and develop skills (including math skills) - but a goodly part of that is 'despite' the educational system: there are teachers who do manage to stimulate the student's natural learning capabilities. The 'educational system' largely suppresses those.
The computer could surely be a useful tool for education - but only if it is recognized that the computer is a support for learning, not a substitute for (thinking and) learning.
As to assessments (of learning; of teaching; of whatever) - well, all of that will develop naturally when we understand how to use our own (adult) minds effectively so as to enable the student minds also to be used effectively.
Currently, we're a long way from any of that.
In this day and age, we still have people claiming that children must be PUSHED to learn - despite all evidence to the contrary that learning is natural for all *normal* children. It's high time we get over such medieval ideas.