I am not advocating exceeding the stated maxima but ...Whatever, 'safety margins' are just that, and we are theoretically constrained to work with the 'maxima' which are stated in the regs, even if those limits are deliberately conservative'.
you seem to be saying that for 2.5mm² the 70° will be reached at 27A and the safety margin is it being able to cope with higher temperatures but ...
could it be that the conductor (2.5) can actually handle more than 27A - say, for example, 35A (I have no idea of the actual figure) - and the 70° would never be reached with compliant installation?