There are a few kids who throw the same velocity from OF or the mound. I think when that happens they have worked on pitching much more and they don't utilize as good of mechanics for the outfield throw.
Nearly everyone should throw with more velociuty from thye outfield. Even some very good pitchers. There are no pitching mechanics that can fully replace the momentum they get running in on a ball and crow hopping. The fast arm gets even faster with the body action involved in throwing from the outfield.
A fairly recent example would be Kaleb Cowart (1st round pick by the Angels a couple years ago. Kaleb was an infielder/pitcher and was drafted as a 3B. He threw easily 90+ across the infield and mid 90s on the mound. He decided to get into a contest we held and threw 100 mph from the outfield.
Obviously being on the run and crow hopping doesn't allow for as good of accuracy as a pitcher requires. But it sure puts you in a position to throw your hardest. Going back on a fly ball is a different story, or even standing still. Best velocity is from fielding the ball with momentum towards the throw and crow hopping with a long arm action. Pitchers just don't have that luxury.