I don't think there will ever be a perfect measure for schools that can take into account every situation of every student. Progress 8 is an improvement on the A*-C measure simply because it no longer encourages schools to focus on a single grade boundary and ignore the progress of the very top and very weak students. It is probably the best measure used in my 20 odd years as a teacher for schools. What should be done by OfSTED is to consistently use the Progress 8 in context for the school - it does happen e.g. the school graded as good despite a very negative progress 8 score due to the school entering students for IGCSE English as they felt it was the most appropriate course for them.