Jeff posted the distribution a while ago: Competitive Mode Tier Distribution
But Overbuff has two problems…
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Private profiles limits the number of account info they have. It’s suspected that higher skill players are less likely to make their profile private but we don’t know for sure.
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More skilled players in general are probably more likely to check their stats on Overbuff so there is probably already an over-representation of higher SR players.
Overbuff isn’t completely useless but take their numbers with a grain of salt. Blizzard themselves are the ultimate authority.