The only real reason for range limitations is targeting capability. Sure you can launch a warhead an infinite distance, but the question is will you be able to predict where the ship will be when it hits, and if it has enough time to react and move the 100 meters to get out of the way.
Even if the weapons were travelling at one million kmph, a ship would have almost 2 minutes to move out of the way. Even a few seconds is potentially enough to make a course correction. (presuming that the systems are automated).
Anyways, BFG isn't truly a representation of spaceship battles. It is a little, but it is more WWI/II naval battles. Even some fleets take more after modern fleets (TAU), and others, more ancient fleets (Eldar)
BaronI, in 'flawed ships' the Dominion isn't a +15 point upgrade for the Armageddon, it's +25. Remember the armageddon got reduced 10 points, and in the case of how I would represent it, it would have only 4 torps, to fit with BFT. Making this more like 30, a reasonable (albeit still low) price for 4lb.
The Victory doesn't need 20cm speed for its NC, the Apocalypse has 15cm and it does fine. Besides, if your enemy can shoot at your NC battleship, you're doing it wrong. (or probably couldn't use the NC anyways)