Summary of some points to keep in mind -
(a) derate/flex meet the certification requirements - then why do we accept going at MTOW in critical conditions ? Double standard ? The days of pilots doing things to make life more comfortable for pilots during flight have long gone to make way for LoCo competition.
(b) derate is the limit so there is no increase beyond that during the takeoff phase
(c) flex can be run up to rated/derated as applicable
(d) if (c) is applied, be VERY wary of dynamic effects on thrust and subsequent yawing moments. I was involved in the investigation of one fatal where this was very likely a contributing factor.
(e) if (c) is applied to a turboprop, be careful of drag effects due to prop blade gyrations - similar concern to doing asy work by snap closure of the throttle - one can get significant drag on the "failed" prop
... as GF observes in a parallel thread, do it slowly ...