You have to distinguish between fix derates and TO derates.
Fix derates are accomplished by changes to the HMU or EEC.
A TO derate is simply selected in the FMC ( I think not every operaror has this configuration)
When firewalling a TO derated engine (e.g. 27K to 25K) you get 27K.
Firewalling a fix derated engine has no effect on the rated thrust.
(B737)
Please correct me if I am wrong.
Michael