Hi Procyon,
I very much agree with your analysis. Unless slugs are extremely small (and I don't understand why they should be), and relative speed is very low (ie the ship overtakes the slug), the amount of kinetic energy will be such that armour will make very little difference. With respect to projectiles, a hit is a kill.
On the other hand, long distances and large speeds would make 'aiming' extremely difficult. Even with perfect control on the firing vector (and there are physical limits on this), you can't expect slug and target speed to be absolutely constant, nor can you expect their trajectory to be perfectly straight (due to gravity, at least). And volleys of slugs shot at long distance would probably scatter a lot.
In my opinion, projectile weapons will make short distance combat impossible (which is why, I believe, missiles, and maybe some projectiles, must be area weapons), but at longer ranges they will become a form of "lottery weapon", with a small probability of sudden kill. And volleys of projectiles would only improve the hit probability at short range (as range increases, so does scatter).
Apart from luck, the only defense against slugs I can think of would be some yet to be handwaved "grav/mag/whatever shield" that could deflect incoming trajectories. This would work against slugs arriving on low angle trajectories, but probably not for direct hits.
Now, this raises an interesting question about missiles, and "explosive rounds" (if such things exist). Should they be fragmentation or "pure energy" weapons? (or maybe a combination of both)?
Francois