Frequencies diffused will not contribute the same at the measuring point, same as for your ears. If you use diffusors to break up some reflections, the measurements will show reduced energy at those frequencies. If you add diffusors to provide additional reflected energy at the listening position, a measurement mic will register the increased energy. Impulse response will be similarly affected, along with decay/reverb measurements and so forth. In short the effects are measurable, and will show up in time and frequency domain plots if you care to make them.
Same is true for absorptive products, natch.
As always, taking the right measurements and interpreting them correctly can be a challenge.
I use measurements to help confirm optimal placement of room treatment.
FWIWFM - Don