Flex/Assumed temp T/O actually have increased margins as the stop distance is calculated on the TAS (assume nil wind) at the higher temp not the actual temp. So e.g at ISA if V1 was 140IAS then the stop calculation is based on 140TAS, if you assume a temp of 60C then TAS used will be higher around 155 (guessing here). V2 margin is increased also as it doesn't take as much energy to accelerate as is assumed at the higher temp. So Missy not withstanding the limitations mentioned by others as to when you can use a reduced thrust T/O, they do not have a reduced safety margin, yes a bit more runway used. Fox3 some jets can reduce their climb thrust as well for the same engine longevity reasons as Take Off. I would say most companies that do this would allow it below certain weights only, so you are not likely to see any huge variety in climb rates as a result. No more a variety than a heavy 340 vs a light one.