I looked at a year's worth of data for that flight and it's significantly (>45 minutes) delayed about 10% of the time, while it arrives on time (<15 minutes delay) about 70% of the time. I think you can be reassured that it is not as bad as you initially stated.

I'm afraid I tend to think that after travelling that long you will be uncertain what time it is, which way is up, or where you are and that adding an hour to the final arrival time won't make much difference to your sense of time and place. I suggest you focus on making your journey as stress free and comfortable as possible by bringing and doing the things that are important to you (in snacks, drinks, cushions, clothing, entertainment, headphones, etc) because you will need that for this journey whether it is 24 or 25 hours long.

MFCC knows their stuff so I'm sure BA are working on it, and punctuality is always important to BA, not least to avoid problems at Heathrow where one delayed departure affects other flights as gate space and staff are not available for the next flight they're supposed to handle and overall punctuality degrades rapidly.