Handbook: Major Periphery Powers (pg. 88-89) set just pre-Jihad very much implies that the MIM was the best intelligence agency in the periphery prior to the Reunification War. Then during the Star League era it was forced to report to the Star League Council and suffered from a lot of budget cuts.
It then goes on to say that during the Succession Wars era, the MIM got more funding and mandates/missions that had been shuttered during the Star League era. That it regained its reputation as the best intelligence agency in the periphery.
It says this contributed to excellent work during the Andurien War of Succession, helping to enable the early victories against the CCAF, which began in the early 3030s, but that work wasn't sufficient to prevent the defeat the MAF suffered.
It then goes on to say that the followers of Kyalla were purged after her attempt to assassinate her daughter Emma. The MIM was then purged of some elements in the aftermath of that, and restructured, and given more funding. It suggests that the MIM was tasked to monitor the Inner Sphere's response to the Clan Invasion in the 3050s, which sent their agents pretty far and wide.
It goes on to speculate about how the MIM got so good, implying that they got a lot more money and talent from the 3040s onward. It does talk about the rumors of the Ebon Magistrate, including rumors that these elite covert operatives have received "personal modifications."
So that book does a decent job breaking it down. Even if some of the things it says are presented at that time as just rumor. We now know a lot of those rumors are fact. It does vaguely reference the rumor/idea that the MoC found some lostech cache or that the MIM gained some mysterious benefactor. Though it does wrap it all up by saying maybe this is just all Emma's doing, that she sought to build up the MIM into what it became and was successful.
It feels to me like they left some doors open to future writers, but also threw a "the simplest solution is the right one" logical answer into the mix, just the idea that Emma sought to improve the agency and threw resources at that priority (an agency that reported ONLY to her by the way, if I recall correctly, General Hadji Doru tried to restructure this and she refused that recommendation, according to FM: Periphery in the MoC section that addresses the MIM). So during Emma's time the MIM got very good.
During Emma's time, she also kept a tight hold on who the MIM reported to, ensuring that it only reported to her. I don't think this is a coincidence. I think those two concepts go tightly together.
