I’ve heard several opinions on preparing French meringues:
Any concrete facts about the efficacy of either method? Does either have more stable foam or with better overrun?
The main thing about French meringue is that it is very prone to weeping. And this is exactly what the sugar addition techniques address.
When you add the sugar late, as in streaming it after it has been whipped to a certain point, the sugar does not do much and is only there for taste. The meringue has large bubbles, a lot of volume, is stiff-ish, and after sitting for a bit, it starts weeping (you get even larger bubbles on top, and a puddle of liquid on the bottom).
When you give the sugar time to dissolve well, it binds the most liquid parts of the eggwhite, and you get a very different texture. It is silky smooth, with small bubbles, and the peaks are prone to trailing, as opposed to being stiff, and is very pipeable. The total volume is less than in late-sugar meringue, and it will not weep even if let out to stand for a long time. Overall, it is very close to Italian meringue in texture.
I actually think that the second result is the one that is desirable for practically all preparations, since I have not discovered a good use for the dry, weepy stuff. I would speculate that the "stream sugar into it" technique is a historical one, from the times when meringue was beat by hand. Then, the beating was a long process, and even when added late, the sugar was able to bind the wetness. Adding it early did not improve the texture, but increased the beating time, so bakers learned to add it late.
With a mixer, I give the sugar time to dissolve. Here, I think that your option 2 - night before - is unnecessary long (although it should work, too) and option 3 - dump together and mix - is too short to work. Instead, I mix the eggwhites and the sugar, and beat them 1-2 minutes on the lowest speed, just enough for everything to mix well and the proteins to start unfurling, but not enough for a foam to start forming. Then I turn off the mixer and let it rest for 5-10 minutes. After that, I turn it back on on the highest speed and let it work until I have the desired stiffness. The result works for pretty much everything.
As an illustration, this is a cherry pie I made, topped with this kind of meringue. I wouldn't have been able to pipe these shapes with a streamed-sugar meringue, and if I did get some kind of shapes there, the peaks wouldn't have kept their shape in the oven.
Personally, this is the sort of situation that I think is fantastic for experimentation. My recommended medium would be a chiffon cake that depends on whipped egg whites for structure. Keeping every other aspect of the batter the same, try each method, bake the cake the same way, in the same pan, and look at the crumb. Is it even, fine, loose, dense, airy? How thick is the crust? How does it feel when you bite down? Crumbly, soft, spongey, firm, dry? That's going to give you the best answer (and an excuse to eat cake.) Alternatively, or additionally, just bake 2-3 egg whites worth of meringues directly to avoid excess. Take some pictures of your experiments if you decide to perform them a few weeks apart.
Generally, this is what I've gathered for myself from various sources and/or experiments and accidents:
The longer you wait to add the sugar, the denser your final product will be. One potential issue is that the sugar might not thoroughly dissolve before you've finished whipping your meringue, leading to extra caramelization on the outside, possibly even graininess, or (depending on the use) a separation of sorts. I've had a few cakes come out with a denser layer at the bottom with a much lighter, but drier crumb higher up.
I can't precisely explain the physics, but you can overwhip egg whites into a grainy mess. That kind of curdling is caused by proteins coagulating too much together. So, the addition of sugar not only adds stability to the meringue, but inhibits the proteins from binding together quite so quickly or easily, and so helps prevent that curdling. Based on this, I think it's reasonable to assume that if you wait too long to add sugar to a meringue, some of the proteins will have already bonded too tightly together for the sugar to properly interfere. As a result, you wind up with some tighter egg protein matrix and some looser, as well as sugar syrup that's not as well distributed throughout the foam - meaning the sugar doesn't always get where it needs to be to stabilize the foam, and instead weighs it down unevenly. Not great texture-wise, and worse than that, unpredictable.
So what's the problem if you add the sugar too soon? Basically, it works too well to stabilize and inhibit the foam's formation. I've done that multiple times because I personally prefer it for chiffon cake. The sooner you start adding the sugar, the more thick and velvety smooth the meringue becomes. That can be an issue because when you bake it up, the texture is very fine. It dries out easily, and where it's not chalky, it's marshmallow-y. I've never bitten into a piece of insulation foam, but I imagine it has a similar texture to a meringue that got sugared at the beginning. Not bad, IMO, and fairly predictable, but definitely not ideal by most standards for most applications.
I've never actually tried adding the sugar to the egg whites the day before, so I can't speak to that specifically. I'd imagine it would result in an extremely fine foam, and lead to a dry or spongey final product as well - either of which might be exactly what you want for a given application.
That's the most difficult aspect of this question to completely address. French meringue can be used in multiple applications, piped and baked to dryness, broiled or torched on a pie or for baked alaska, as a last minute dairy-free sub for whipped cream, for pavlova, as the non-chemical leavening in cake, a base for a buttercream... The desirable amount of sugar and best time to add can vary by application, and more importantly by person.
I'd imagine various people swear by their own methods for their own reasons, possibly because they favor different textures, or different desserts entirely, or their mixer runs at a different speed, or their preferred bowl is a different shape, both of which could be significant. So again, I really think this is the kind of question you're best off answering through direct experimentation. There is no one best method, any more than there's one best cake.
