but I was sure that some of you would be able to throw some light on the expected behaviour.
nobody (who knows what he does) has been stupid enough to use components outside their limits and thus nobody (who knows what he does) will be able to answer you.

I can give you sort of an anwer by telling you that there once was a post "I have been overclocking the chip for years, why does it not work now?" Well the answer is simple: "working outside the limits may appear to work till it show you that it does not.

BTW if your customer specify "must work to -40C" and you have accepted that in the contract, you will be liable to have your behind sued if you use any components not rated for that temp.

Erik