IMC has been the bane of my existence more than once.  So here is my take from experience backed up with some lab evidence.
I have had failures attributable to insufficient IMC.  Almost all of these were corrected by either ramp-to-peak or increased time above liquidus / intimate contact time / temperature.
For SAC, the intimate contact temperature I have been able to dial in on is time above 208C after liquidus is achieved.
For SnPb, the intimate contact temperature I have been able to dial in on is time above 177C after liquidus is achieved.
I have found that if I can back end a little more time above these temperatures on the profile, I have more consistent joints.  I have not devoted a whole lot of 'science' on these numbers just 20+ years of trial and error coupled with white papers, IPC BOK, and other research.

I have yet to have a problem or a return tied to excessive IMC.  I am sure that at some point excessive IMC may cause or have caused a failure mode, I just have not seen it.  The few times where it may have been a contributing cause, I also cooked the parts making cross-sectioning of the IMC moot.