This means we can now get a rough estimate of how accurate Overbuff is, using their weekly data:
Bronze = 9 right (Orisa instead of Ana), 5 in position (D.Va, Mercy, Rein, Genji and Hog right)
Silver = 9 right (Zen instead of Genji), 4 in position (D.Va, Moira, Mercy and Ana)
Gold = 9 right (Zen instead of Soldier), 3 in position (Mercy, Rein and Lucio)
Plat = 10 right, 4 in position (Moira, D.Va, McCree and Soldier)
Diamond = 10 right, 5 in position (Moira, D.Va, Mercy, Ana and Tracer)
Master = 8 right (Rein and Lucio instead of Ana and McCree), 1 in position (Zen)
GM = 8 right (Rein and Sombra instead of Mercy and McCree), 2 in position (D.Va and Tracer)
Overbuff is mostly right with it’s 10 hero compositions (63 out of 70 heroes correct is brilliant), and at 24/70 in the right position isn’t too bad for accuracy either. Notably in the 7 occasions where a pick was outside their top 10, only 3 of them were over 0.5% away from the top 10 (Ana Master = 0.55%, McCree Master = 1.18%, Ana Bronze = 0.57%).
In general it would be preferable to have the official statistics readily available, but in the absence of that Overbuff provides a good basis to work from.