Funny enough, in testing in a Gryphon fleet vs triple [REDACTED], the best Legion build I've found so far is...4 Cobras, 2 Gauss, 4 HVD, 1 Heavy Mauler. Note that this is mostly a long-range, standoff engagement due to the Gryphon fleet, and a longer battle since it's triple [REDACTED].
* I couldn't find any medium missiles that could provide the same amount of damage over the long haul compared with just putting ballistics on those medium composites. Even with EMR and Missile Spec. Maybe they'd be better for shorter battles, or if the battlespace weren't dominated by masses of missiles from the Gryphons.
* I feel like Ballistic Rangefinder for the small ballistics is a bit of a trap. Yes they allow you to boost their range to 900, but you're spending a lot of OP on it. The total damage dealt to OP spent ratio wasn't that good. A Light Needler with Ballistic Rangefinder did roughly 75% of the damage of an HVD, but is much more expensive due to the extra hullmod. Perhaps if the Legion closed in more then they'd be more useful (the Legion flagship was mostly autopilot, except for occasional burn drives to keep it at the front when it got blocked by Gryphons).
* Yes the ballistics are very kinetic-heavy, but they have relatively good hit strength so they do decent damage to hull. They ended up putting out more damage overall than sticking HE weapons in those slots.
The build does around 16-18% of the overall damage of the fleet, while Legion XIV (4 Cobras, 2 Squalls, 4 HVD, 1 Heavy Mauler, 4 Railgun) does around 20-22% of the overall damage. In comparing the XIV versus the regular version, Squall does far more damage than Gauss (or any other large ballistic that I tried) with zero flux, and although Legion XIV needed to get Missile Spec and EMR for that, it didn't need to load up as much on vents since there's not as much flux usage. Hence it could afford to slap on some Railguns for extra damage without breaking the flux bank.
So basically, regular Legion has roughly 80% the damage output of the Legion XIV, as far as I can tell. Unless someone wants to suggest a better build. I'm not sure how much of an improvement adding 20-30 OP to the ship would actually do though, in terms of allowing it to increase its damage output, for example.