The primary factor in raw combustion efficiency is the return water temp, but firing rate is a strong secondary factor. The sweet-spot for most mod-cons is about ~1/4-1/3 of full-fire. Below about 1/4-fire the turbulence on the fire side of the heat exchanger drops dramatically & quickly, leading to a laminar-flow, increasing the stack temp. At full-max the turbulence is good, but the velocity is higher, with greater volume of gases blowing by more quickly than the water can condense out. Different heat exchanger designs will have different firing ranges where the efficiency peaks, but no mod-con heat exchangers are designed for peak efficiency at full-fire, since the design-presumption is that it will be modulating at part load the vast majority of the time.
It looks roughly like this:
With suitably low return water temps for condensing that difference is less than 5% though, and with return water higher than condensing temps that difference is under 1%.