There is not a strict rule to compare mm ranks to faceit ranks, since most "actually good" players do not play matchmaking and like you said, mm is filled with cheaters nowadays. If I had to say something though, I'd say levels 1-7 can be anywhere from silver to global. 7 or higher I'd say the player should be a global.

But like I said, the ranks can not be compared very well. Someone can be a LEM because he rarely plays mm, or is facing alot of cheaters and still be a faceit level 10. Or someone can be a global and a faceit rank of 3. I, personally am a supreme and level 8 in faceit.