I have a spectral power distribution (SPD, 5 nm steps) for all light sources in the scene and SPDs for the reflectance of all surfaces under any light / viewer angle in question.
I'm going to calculate in floating point precision and not clamp any values. Except for pathological cases like monochromatic light or special things like dispersion effects, does it make a difference whether I render using the spectrum, XYZ color space or linear RGB color space? Note that I will convert the result to sRGB as a last step in any case.
Specifically, is it invariant at which point (before or after multiplication of surface BRDF with light intensity) I integrate to convert spectrum to XYZ? Does the introduction of reference white in XYZ / RGB distort the colors when working with light sources of a different color / SPD?