Your max assumed temp is either
a) perf-stop limited
b) Perf- go climb limited
c) Perf - go obstical limited
d) Perf - go all engine 115% limited
e) regulation 25% max limited
f) some other limit
If you set the wrong one you will either break the rules or break your aeroplane.
Some systems will prevent you setting more than the 25% reduction but thats not much protection.
Your selection, training and operating policies are all geared to prevent you setting the wrong temp, as well as shutting down the wrong engine or turning the wrong way or flying at the wrong height or speed in the wrong configuration etc.etc.etc.
Failure to do any of them correctly could have the ultimate consequence.
I wouldn't worry about how much more runway you may have used. I would worry about why the wrong temp was set and how to make sure it doesn't happen again.