At the 2004 Flight Operations Symposium, there was a B737 presentation on high altitude turning. I understand an aircraft entered low speed buffet at high altitude when turning in heading mode. In LNAV, the FMC will take in to account the thrust available whereas in HDG mode, the angle of bank is only limited by pilot input.
It was presented that the FMC can only calculate the MAX ALT properly if it is given an accurate weight and cruise temperature. My recollection was that the point at which it transfers from predicted to actual temperature was at TOPC but it may well be more complex than that.