So called "limp mode" can be triggered from many things. Actually, according to my SA, what BMW calls "limp mode" is an extreme condition that includes (besides reduced engine power) the AT locked in 3rd gear (I don't know what would happen with a MT). If the message just says "Reduced Engine Power", then it is not technically "limp mode" and my SA and I were talking at cross-purposes until we got that sorted out.
That said, both coolant and oil temps can cause reduced engine power and it goes in steps from 90% to 10% according to how high the temps get. In my case I had a stuck Vanos solenoid which triggered the power reduction (nothing to do with coolant or oil temps). I was still able to keep up with traffic in all cases but my 335 performed more like a base model Honda Civic.