Re: The Legion only having 60 more OP than the Dominator: thats true, but fighter bays essentially "count" as 10-15 OP each in their design, so its more like the Legion has 100 more OP. Plus the Legion has higher max dissipation than a Dominator (same base, but 50 vents instead of 30), better range, more missiles, and the large mounts are turreted rather than hardpoint (both an advantage and disadvantage), and more armor/hull/flux capacity. So its like a Dominator + 4 fighter wings... only better in almost every way. The only thing a Dominator has going over a Legion is a better layout of its small mounts for point defense (particularly the rear) and being burn 8 for cruiser fleets. Legions absolutely destroy Dominators (as they should, being 40 vs 25).
For large ballistics being an advantage: its because large ballistics are a significant step up over medium ballistics, but large missiles aren't that much of a step up over medium missiles. The large missiles are really nice its true, but medium missiles are great for being medium mounts, so the value is nearly the same even if the specifics chosen are different.
Consider just a basic Mk IX + Heph/Mjolnir/Hellbore combo/mk IX combo: its got better range than medium mounts (except HVD/Maulers, but those are only 100 more range for being much less efficient and low DPS/mount and per OP), better shot size, and better damage per OP. Now add 5 annihilator pods/typhoon reapers and compare to a Legion XIV with 2 Hammer Barrages/Cyclone Reapers. The XIV has slightly better torpedoes but honestly not that much, but much worse guns. Locusts and squalls provide options that the medium missiles can't really cover so those are good, and squalls in particular are excellent now that they have more range, but 5x Harpoons is not that much worse in terms of guided HE options and carries significantly more burst potential than Hurricanes at the cost of total ammo. The XIV is better mainly because of it being XIV and getting more OP/armor/flux, rather than mounts, in my opinion.