I don't think there is any chance of AT being accepted in a Court as justification that your rifle was shooting over power.
It's a simple matter to remove the AT, increase the power and then replace the AT.
Because it's an absolute offence it doesn't matter why it's shooting over power, just that it is. We know full well that AT is not going to stop anybody putting their rifle over power, should they wish to, all it does is inconvenience the law abiding and cost them their Warranty.
A standard test has benefits for everybody, even the Scottish ACPO could see the value in it.
Manufacturers could set to it, we could ensure we comply with the law with it and the Police could prosecute with it. Difficult to see why it can't be done with so many benefiting from it.