I'll second that buff, 14 per shot for a hvy slot is....something. Considering ufo armor doubly so.
Even the Hvy plasma craft weapon isn't in a top tier spot. It takes 4 minutes longer to fully unload then an obliterator cannon for only 25% greater maximum potential dmg, and suffers more from armor. 5% better accuracy and 5km longer range(for plasma) is not much of an edge when pilots and craft bonuses are considered.