After a couple more night thinking about the rogue trader macrobatteries vs lance weapons issue... if I were to run a space combat in 40k RPG games (rogue trader, dark heresy, deathwatch), I'd use the following additional rules in regard to macrobatteries.
Scattering
The further the target is away, the less likely the firer can achieve the saturation necessary to score effective hits. After all, space is big. Really big. Mind-bogglingly big. Firing macrobatteries point-blank means smashing most of your projectiles right into the enemy side. Conversely, firing at long range means there are fewer projectiles to occupy relatively the same amount of space.
When firing at a range beyond the Macrobattery listed range, in addition to -10 Ballistic Skill test (p. 220, RT corebook), the effective strength of the macrobattery is halved (rounding up). Alternatively, it takes 2 degrees of success to score additional hits, instead of 1.
Lance weapons don't suffer this drawback.
The goal is to maintain the lethality of macrobatteries at their nominal range, but make lances superior at long range. With the half-strength, or extra degrees, the ship's armor really makes a difference at long range, whereas lances will be able to cut right through and not even notice. This turns lance weapons in opening salvo, snipe and kite weapons. Since it's a giant laser in space anyway...
Thoughts, Darren?