Hullo Rich......I certainly do not qualify as...
'one of you awfully clever chaps' ...
However believe there is an explanation for the term you describe.......
If an engine is rated to 1000HP, that is based upon many factors, and one of them simplified is its duty rating
So if the engine is rated at 1000HP at 100% duty....then every mechanical component is designed to perform at its rated output also for a given time period
However due to parameters, if you increased the fuel and oxygen beyond its maximum rated requirement, by increased combustion and over speed you would exceed the original 1000HP output
...however at a potential cost of overheat
I suppose our electrical boffins could offer similar scenarios with amps & current draw exceeding the design & thereby also with the same result of overheat
Apart from that, I had always assumed that any form of throttle in a nuclear reaction would be to increase cooling, therefore reducing the output power
In conclusion, it would appear OK within the scope of this English language to suggest that an engine could perform at 110%........however I find a fundamental flaw in any suggestion that a human being can perform at any value greater than 100%
So that's my twopence worth from one very poor old pensioner to another
...... Derek