That isn't the story I am told from the few guys around that actually ran them against the competition.  Where I am at 5020s were popular with the IH 12 series right behind them.  I would love to own a D21 but my overall pick for the 60s era turbo tractor would be the IH 1206/56.  We also have a D17 from the mid 60s and I don't see what tractor it would out perform either.