This seems very good Benzene, the only thing I think I can see from the difference (from how I understand the English and the Italian) is that maybe there is a better way to put high-profile than significativa.
I get the impression significativa represents more of a quantity of crime, whereas high-profile is more to do with big cases that make the headlines / news reports.
High profile Mafia crime would be like, what happened to Paolo Borsellino and Giovanni Falcone back in the 90s.
In another thread titled "high profile", distinta was suggested, and I see jh344 has used this here, is this not correct?
According to Alxmrphi I'd say:
Dai primi anni 90 (del secolo scorso) c'è stata una diminuzione dei grandi delitti di mafia.
Dai primi anni 90 i grandi delitti di mafia sono diminuiti.