Maybe we can extend that question to the B777. I have noticed several times that max/opt alt indication increased significantly after the step climb was completed. That would have meant that the ISA Dev must have decreased significantly over the temp that the FMC had used for its calculation.
That would mean that the FMC displayed opt/ max alt can be off significantly if the temp at higher altitude is lower than actual alt temperature minus 2° per 1000'.. right?