The basic measure of the instrument spectral response is the photon conversion efficiency (PCE), which shows the electrons out of the detector per photon into the instrument, as a function of wavelength. The derivation of the PCEs for all MIRI modes other than the coronagraphs is described in a technical note. The PCEs themselves are available in Excel spreadsheets for the imager, moderate resolution spectrometer, and low resolution spectrometer.
In many situations, one wants the relative response of the instrument in units such as electrons out per Watt of photons into the instrument. Since the energy of a photon is inversely proportional to its wavelength, the PCEs need to be weighted in proportion to wavelength in these cases. This adjustment is significant in the relatively broad bands of the imager, where a Watt worth of photons at the long wavelength end of the band will contain more photons and produce more electrons than a Watt worth of photons at the short wavelength end.
Performance estimates for the instrument plus telescope can be found in the relevant MIRI encyclopedia paper. These values can also be used as worked examples to be sure that the PCEs are being interpreted as intended.