I can see the argument you're putting forward Paul and no doubt it is worth consideration.
The big thing though is that you are unsure, and I think that has answered your question.
You'd be happy thrashing round a track on tyres which still had an obvious amount of wear left in them.
Even if I told you it was safe, would you REALLY be confident in doing the same on tyres at the end of their service life?
And what if the track day is a bit damp?????
Just my thoughts................