A Tier 15 weapon against a Tier 18 target would cause 7% barrier face damage and "light" hull damage according to the DRv3 wiki article. A Tier 15 weapon would have to fire fifteen times at the same spot to even get at the hull, assuming the target's barrier power was not supplemented from any of the other five probable facings under DRv3, and would face similar difficulty achieving any meaningful damage against the hull even once the barrier was down. One could amass every vessel capable of Tier 15 firepower in their fleet and have a high probability of accomplishing nothing before they were fully wiped out by the target's defensive weapons. The one upshot is that a Tier 18 ship could only have a maximum of 16 Tier 15 weapons, which might slow things down a bit in larger engagements provided the Tier 18 thing is fighting alone. Still, for an RP that is supposed to at least allow for a chance at victory in engagements based on player action, the extreme set by the math doesn't sit well with me.
By contrast, compartmentalizing into multiple Tier 15 'sections' would allow for something more realistic that makes concentrated fire practical, albeit very risky with the target surely having insanely powerful point defense on the scale of gunship armament on the low end(Multiple Tier 15 sections can have far more weapons than one Tier 18, but spread across the hull as would be prudent for high-grade point defense). Crews could still try to resist concentrated fire through RP by diverting barrier power from nearby areas to a degree as well. As for hull damage, the hull is unlikely to scale linearly with size past Tier 15 and overall durability would be more a function of just how much target there is to damage before it is fully incapacitated.
Armament should scale properly with surface area, which the DRv3 system can't achieve properly with the massive leaps between Tiers 15, 16, 17, and 18. Number of barrier facings should also scale with surface area, rather than making one insurmountable set of six barrier facings. Hull thickness also will not scale linearly with size. The more I think about it, the more I feel that splitting superstructures into deadly and mutually supportive Tier 15 "sections" is the way to go. DRv3 just falls apart at these scales otherwise.
I'm trying to balance 'risky but remotely possible to damage with concentrated fire' due to an increase in general armament against a hypothetical Tier 18's 'virtual mathematical invulnerability'.
Edit: Replaced 'negligible' with 'light' due to an error.