Well, I've played with them all and the default one with slower spinup is definitely the strongest one. The others are balanced better, the one with -40% damage is interesting as it really makes the weapon into a support weapon used for leading focus fire. You won't really kill stuff quickly but the very apparent slowdown is still quite useful combined with a decent team.

[...] You can produce a similar effect to the slow by adding an increased knockback mechanic. It probably wouldn't require any other negative effects because sometimes you wouldn't want to knock an enemy away whilst you would in other situations. For example, you would want extra knockback to hold off an enemy uber or to push a scout attempting to attack your medic away but you wouldn't want extra knockback when trying to finish off enemies attempting to retreat.

Remove it entirely. The problem isn't that the stats on the weapon are wrong, the problem is the horrid effect it produces, which no tweaking of percentages will fix.