But as you've already discovered.. STC breaks precombines right off the bat. And I'm guessing STS does exactly the same thing.
Well, my first attempt to test STC suggested that it was actually working as advertised. I had the "triangle of death" all upgraded to level 3, and the Xbox One X was already buckling under the load. It wasn't unplayable, but it was clearly struggling. I was sure that if STC really did just disable precombines, it should destroy performance in the triangle of death, and likely crash the game. But when I tried, there was no noticeable impact to performance, and everything worked just as it did before, except everything was now scrappable.
I suppose it's possible that STC is working in some parts of the map and not in others, and that the problems in downtown Boston could be rectified, though I'm not holding out so much hope.
One way to check is to get a settlement expansion mod that expands a settlement well beyond the current cells. If STS is breaking precombines right off just in the original settlement cells, it will stop working outside those original cells.
If the speculation about how STS is supposed to work is accurate, then STS probably can't scrap any object that it wasn't specifically set-up to scrap, because it's actually scrapping whole precombines and then respawning some of the pieces individually. Since it wouldn't have been set up to scrap precombines outside the settlement borders, discovering that it can't wouldn't tell us anything that we wouldn't expect.
There is one possible test, but I'm not sure how to do it yet. If I'm not mistaken, then precombined objects share the same loading pop-in, i.e. the whole precombine should appear at the same time. When precombines are disabled, then each constituent object gets loaded in separately and so their pop-in isn't synchronised. If so, then it seems like it might be possible to visually test for disabled precombines by contriving a way to get ahead of the loading, perhaps by installing a mod that increases player speed way beyond what the engine can reasonably handle. Then you could watch objects pop-in and see what happens.