We do allow for temp inversion in so far as we aren't allowed to use a reduced TO power setting if there is an inversion.
Just seems weird to me that for years and years nobody has mentioned that you have to take into account low cloud in a Perf calc. None of the worked examples given in our manuals makes any reference to this factor and I would have thought this would have been a classic gotcha and highlighted over the years.
Does the Perf calc end when the aircraft is climbing away clean?