Two points - when BA substituted 747s for the Tristar that had a "heavy" landing, the first one provided was P & W engined - it only just got off from MPA, and at Ascension, because BA insisted on day time flights only, its take off from ASI was very marginal, in fact the Fire Crew commander had the fleet on the run at three quarters of the way down the runway.
Secondly, here in France I have seen gliding for the day cancelled when it is very hot (31C here today, forecast 37C tomorrow) because the tug cannot get the tow off the ground in the take off distance available. Also it can be unbearably hot in the aircraft