“But the HN is super more efficient!” You say. And that only matters if we’re shooting exactly at (or under but close to) our flux dissipation... which we don’t want to be doing, we want to be shooting over our flux dissipation with both of these guns. And we want to be shooting as much over as we can in general with both of those guns
This is false. Efficiency is critical when firing over dissipation because it determines how much damage the ship can output before fluxing itself out. Onslaughts don't have infinite flux and will usually be near max flux by the time they finish off a significant enemy. Not only that, but the AI behavior of switching off weapon groups depending on the ship's flux level means that the AI will turn off weapons sooner with less efficient weapons because they have driven the flux up faster. Less efficient = less damage dealt until the weapons turn off because of the flux load.
If the ship can destroy the enemy completely before that happens, then the net flux advantage of flux dealt to enemy vs flux dealt to self is all that matters. Onslaught vs destroyers can do that, and maybe Onslaught vs poorly shielded cruisers. But that just isn't the case with any of these ships vs big enemies.
The net flux analysis (which is deeply flawed anyways, see below) of HN vs Mk IX is to me not a point in the Mk IX's favor. They have equivalent net flux change between the firing ship and the target... if the target has 1.0 shields, which is only a subset of enemies. And the Mk IX's reduce the firing ship's remaining flux pool 200 flux/s faster per gun(important!!). And the Mk IX's take 3 more OP each. And they take large slots that could be mounting Hellbores or Hephaestus. And this is assuming the Mk IX has equivalent accuracy to a HN (which again I don't have hard numbers for, but I doubt).
For completeness, here's the net flux analysis for good shields:
Examining the Mk IX vs good shields (.6). It does 350 DPS, IE 2*350*.6 = 420 flux/s to the target. It costs 400 flux to the firing ship. If we are talking about firing over dissipation, IE converting flux pools, then the gun
barely helps the firing ship, giving a net flux advantage of 20/s, and large well shielded enemies may have deeper flux pools than the Onslaught/Dominator, making firing this gun near worthless (which is not true in practice, see assumptions below).
Under the same conditions, a HN does 250 DPS: 2*250*.6 = 300 flux/sec to the enemy while costing the firing ship 200 flux/second. A 100 flux/second advantage per gun, and a 1.5:1 ratio in terms of flux pool trading (which is not true in practice both because of dissipation and because of other guns firing that compete for the flux pool less efficiently).
I'll make the assumptions of this very clear: firing high over dissipation so that this is comparing the firing ship's flux pool vs the targets (which is always a false assumption, but is at least the limiting case for analysis purposes), against a .6 shielded enemy (not all enemies, but also not uncommon, including the nastiest enemies (Paragons and Radiants)), with all shots hitting (false), and having no incoming fire/armor tanking so that the firing ship does not build up flux from damage (false again).
There's a lot of false assumptions going into the above, which is readily apparent because in actual play because yes its worth it to fire Mk IX's into good shields because dissipation exists, but it both shows how important efficiency is for shield breaking when we talk about firing over max dissipation.... and also shows that this analysis is woefully simplistic to the point of being misleading.
Here is my best attempt at a not misleading piece of analysis:
For a given flux budget, whether that is capacity based OR dissipation based, HN's do 43% more shield damage, given full accuracy. That is not the
complete picture because other facts matter, but it is a true statement. Does this statement matter? Not always. Its only an important statement if a ship is constrained by its flux more than it is constrained by other factors. I contend that the Dominator and Onslaught (but not the Conquest!) are constrained by their flux budgets more than any other factor, in both capacity and dissipation. If so, the above statement is important for the Dominator and Onslaught (but not for the Conquest).
The Mk IX's performance against 150 armor for skilled enemies (turns out the skill and base hull don't stack, which I didn't know until the recent testing in a thread) is a point in its favor for doing hull damage. But that makes it a weapon with a decent secondary role (hull damage) that is outcompeted in its primary role (shield damage).