Since it looks like SS might be updated soon-ish and I've managed to get myself hopelessly addicted to this game (and its mods) in the past month, I might as well make an account to put in my 2c. First, thanks for this mod, it's a really sweet expansion! I'm particularly enjoying the Machete laser, Swift Launchers, and the Piranha (LP) bomber (bombers with Hammer torpedos/missiles seems like so common-sense to me in hindsight that I wonder how no one came upw ith that before). I wish I could find any Lysander DDs or the Starlifter Platform in my current campaign play-through but that's certainly my fault for having a dozen different mods on at one time.
I'm a bit confused on the Scattergun myself. Comparing it to the Devastator's odd IMO since the latter is a quirky point-defense flak gun, but I came up with a similar conclusion on my current campaign but comparing it to the Hephasteus Assault Gun.
Although keep in mind that I slightly altered a few specs with the base game (increased the Officer & Player level cap, as well as the max # of ships possible in the Player's fleet), have the fleet battle size setting set for larger battles than normal, have only been playing for ~1 month, and have exactly 1 post to my name, so feel free to take the entire thing with a mountain of salt.
Basically, my experience was that I looted some Sunburst Scattercannons from some large fleet, used them myself on some Capital ships for a while, but ended up substituting them for the vanilla Hephasteus Assault Gun, didn't miss 'em at all and with the spare 8-24 OP I think I ended up with overall better designs.
If I could sum up all my observations into one overly-simplistic sentence, it's this: The Sunburst is a Hephasteus Shotgun. The damage per shot is similar and both are HE weapons. The biggest difference between the two is the Hephasteus delivers its damage as a stream, like a big machinegun, and the Sunburst delivers its damage in... well, bursts, bursts of space buckshot every 1.5 seconds. In theory the Sunburst should have a slight advantage for bunker busting due to slightly higher projectile damage if I'm understanding the intricacies of armor on the wiki correctly, in practice I found the Heph's slightly higher DPS to ultimately make that difference up so I didn't notice any significant difference in performance for blowing out plates. Coupled with the Heph's lower flux costs, slightly more forgiving flux profile (steadyish stream of flux for firing instead of a modest jump per blast), and a 900 unit range (typical among ballistic weapons), IMO it's more versatile and a cheaper alternative, and made the Scattershot really hard to justify in my configurations no matter how much of a shot I gave it.
If it helps at all, the ships I tried with the Scattershot include the Underworld mod's Infernus(?)-class Battlewagon on the front-right large hardpoint and this mod's Vendetta BB. I never even bothered mounting it on an Onslaught as that hull is choking for flux as it is.
Again, just my observations, I'm loving the mod overall and I'm looking foward to seeing what else you cook up.