Pardon me, had 3 pages to catch up on.
As mentioned already, Belknaps with 5" and 3", and Swift boats with 3" and 40mm.
That said, there's an argument to be made for using the 76mm Sovraponte mount
as a CIWS. The Italians certainly see the thing that way. It gives you DART guided and PFFF/canister type rounds to use in the mount, plus massively better range of 16km for the PFFF. All in a mount
roughly the same weight as a Phalanx!
Canister out of a rifled gun doesn't work well. The rifling tends to induce a "donut" shaped pattern, with few-to-no pellets in the center. You'd need something like the French 105mm HEAT round's ball bearing driving band to de-rotate the canister.
The Italians regard the 3" as a CIWS, not a major gun.
Remember that the Asian Burkes are beamier (and longer, I guess) than the OG.
Burke: 66ft/20m (505ft long)
Kongo/Atago/Maya: 69ft/21m (528/541/558ft long)
Sejong: 70ft3/21.4m (545/556ft long)
ASEV: 82ft/25m (620ft long)
It honestly sounds like DDGX is going to be between a Maya and an ASEV.
I'm leaning towards option 1, personally.
I believe that they're Ship's Service generators.
Diesel engines have good fuel economy across a range of loads, while turbines really only have good economy at a very narrow range of load settings. So for a ship, you run turbine generators for your base load, and diesel generators for the variable load on top of that.
I remember some articles when the Burkes came out, that the beamier hull compared to a Sprucan or Tico had better seakeeping.
I suspect there may be a dedicated anti-hypersonic Standard developed out of the 21" SM3 propulsion stack with a different KV on top. Which, of course, is still within Mk41 thermal limits.