We used to use the Stalker Pro Model that would give both speeds (release and home plate).
5 mph difference would be very good in most cases. Usually we would see 6 to 8 mph difference, some even more. The ball is moving at peak speed out of the hand and it starts losing velocity immediately. Good sinker ball pitchers rely on this lose of velocity, to an extent. In the "old" days of the gun, readings were not taken directly out of the hand. So you would see an 86 and that was the equivilant of a low 90s on the gun (velocity out of hand) that is most often used these days.
This brings up something that has always caused me to think. We have seen outfielders throw 94 from the outfield and the ball seems to lose all its velocity before reaching the target. Another outfielder will throw 92 from the outfield and the ball carries and has a lot of velocity at the target.
I know a lot has to do with the grip, release and rotation of the ball. Therefore it's not possible to grade outfield arms based on velocity alone.
It would be very interesting to know which pitcher has the best late velocity. It's possible it is someone other than the pitcher with the best early velocity.