The difference between assumed temperature and a derate is very significant. Once you derate the engine from 27k to 24k this 3k is not available to you anymore. It has become a 24k engine. In my company we don't use derates, only assumed temp. I am not sure if firewalling the thrust levers would deliver 27k.
With the assumed temperature method, the full rated thrust of 27k is always available.
At least for the plane in you Nick not completely right. Even if you derate your engine you still have the full rated thrust available, you can easily get it by firewalling or just doing a second tick on the TOGA switches.
However if you derate you assume that the thrust available is only the derated thrust and therefore you calculate all performance figures based on that thrust rating. That has some pretty serious implications as of course all control speeds (VMCG/VMCA) are based on less thrust. So you can increase the thrust to full rated thrust, however you might run out of control as the VMCG/A for the derated setting is lower than that for full rated thrust, therefore it is a big nono to increase thrust beyond the max for your derated thrust setting.
With good training that is actually a non-issue, but you have to be aware about it. We do use a mix 737 classic and NGs (and a mixed airbus fleet, 319-330), however we use derate only on the NGs although it is available on the classics as well (FMC is the same). Dunno how they do it on the airbus fleet, but im sure they use at least something similar as well.