I haven’t done it personally (duo with someone to pull me up into higher MMR matches) but MMV seems extremely volatile. A few wins or losses can easily equate to hundreds of MMV in one direction or the other depending on wins or losses.
I’m curious as to what happens to someone’s MMV when they are brought up into a much higher game, win and lose. The character doing the boosting has to be a throwaway alt basically. I’m pretty sure people wouldn’t want to take a hit for losing a match hundreds of MMR below their CR on their main.
What I imagine is that boosters actually sell MMV, not CR. The CR comes with high MMV since you won’t lose CR for losses until they start to converge again. So, buying MMV injections would be the way things are done.