Below 30 degs the engine is able to produce more than max (e.g. 27k). At 0 degs C it may be able to produce 30k (as an example only) without exceeding any internal limits, like EGT, fan speed and whatever other limit that an engineer may tell you about.
However, the max allowable thrust also depends on other things, like e.g. engine mountings (probably a lot of other things too, which again an engineer may know lots about).
Are you sure about this? I assume At 0 degs C engine can explode from overpressure or compressor blades will bend/fail ect. That's why it's flat rated?