Great topic Polonius, I very much appreciate the effort you put in it. I wish it will develop as the "official" 8th edition Astra Militarum discussion thread. On my side I will try to put as many meaningful contributions as I can.
Here are some simple probability results (
mean number of damage) that I already put in the other 8th edition thread. It is now more complete with the addition of hot-shot volley gun. I think it has its place here to give some perspective about tempestus scions load-out and what you can expect from one of these powerful squads.
Take a half-size scion troop squad with 2 special weapons. Give the sergeant a plasma pistol (in order to have him being simply useful, the hot-shot pistol is a total garbage). I count the damage done on the turn they drop between 9.5 - 12 inches of a big target, while they fire all their weapons at it. I do count the 2 hot-shot laser guns as contributing to the firepower. On overcharge, I assume there is a tempestor order to reroll

.I have put also the humble grenade launcher to stress out the gap between this weapon and plasma guns which costs only 2 points more.
vs. T'au Piranhas , ork warbuggies & Co (T5
Sv 4+) :
squad with grenade launchers : 2.000
squad with hot-shot volley guns : 1.926
squad with plasma guns : 2.593
squad with rerollable overcharged plasma : 5.099
squad with melta guns : 3.926
vs. Killa kans or t'au commander (T5
Sv 3+) :
squad with grenade launchers : 1.556
squad with hot-shot volley guns : 1.556
squad with plasma guns : 2.148
squad with rerollable overcharged plasma : 4.667
squad with melta guns : 3.778
vs average tyranid monster (T6
Sv 3+)
squad with grenade launchers : 1.185
squad with hot-shot volley guns : 1.407
squad with plasma guns : 2.000
squad with rerollable overcharged plasma : 4.494
squad with melta guns : 3.630
vs. Aeldari War Walker, Drukhari vehicles (T6
Sv 4+ i5++)
squad with grenade launchers : 1.333
squad with hot-shot volley guns : 1.333
squad with plasma guns : 1.630
squad with rerollable overcharged plasma : 3.630
squad with melta guns : 2.519
vs Carnifex, Wraithlord or the average tank (T7
Sv 3+)
squad with grenade launchers : 0.870
squad with hot-shot volley guns : 1.315
squad with plasma guns : 1.537
squad with rerollable overcharged plasma : 3.414
squad with melta guns : 3.537
vs Riptide (T7
Sv 2+ i5++)
squad with grenade launchers : 0.630
squad with hot-shot volley guns : 1.000
squad with plasma guns : 1.963
squad with rerollable overcharged plasma : 3.093
squad with melta guns : 2.556
Tough tank like Vindicator (T8
Sv 3+)
squad with grenade launchers : 0.778
squad with hot-shot volley guns : 0.778
squad with plasma guns : 1.074
squad with rerollable overcharged plasma : 3.414
squad with melta guns : 2.667
Houston, we've got a problem !
Plasma guns are so much better than grenade launchers for just a little more price. They retain their efficiency better against big targets.
Hot-shot volley guns are not that bad, they are on par with grenade launchers sometimes, and on some occasions clearly ahead.
Still, they do not justify their high price compared to plasma guns even on standard mode.
Plasma guns on overcharged mode with rerolls consistently outshine meltaguns in all but one scenario at their job of chasing down tough stuff.
What the developers were thinking when they made plasma guns only 7 points ? This thing needs to be at least 10 points !
Overall at current prices, I see no reason to not spam plasma in scions squads. They are better than any other weapon and very versatile, except maybe at horde control (which I will look at later).
Don't forget to sprinkle your army list with tempestor primes for these crucial rerolls of

.