I think that the time to measure the accuracy of PIOMAS predictions against reality is in mid-September. Even then we might be taken by surprise if we rely on it. We can only pinpoint inaccuracies when the model predicts >0 ice depth, and there is observed to be no ice.
There have been many references in this forum and others to experienced researchers on the ground observing that the ice over much of the arctic is qualitatively very different to what it has been in the past. Viz: This TED talk - , in which David Barber relates a cruise through 2M thick ice at 13 knots in a ship which can manage only 13.5 knots in open water.
The calculations underlying the PIOMAS model predate whatever changes to the mechanisms governing the growth and melting of arctic ice have caused these observable phenomena. The same is true of most of the submarine and other observations which were used to validate it.
The latest possible direct comparison between the PIOMAS model and reality using the link at the top of this thread is at a point in time when most of the surface has a model-predicted thickness > 1M.
So if the model is overestimating the average thickness by 99CM, or if it has the thickness right but the composition of the last remaining ice is such that the model cannot predict what will happen to it, or when, we wouldn't know.
My guess is that for the last 1M, at least, PIOMAS is as clueless as the rest of us.