Thanks for the feedback, folks.
Loris, you're close - all of the Aerosil variants that are hydrophobic contain an "R". There's no Aerosil R 200, but there is an R 202. And Barry's experience is similar to my own.
Robert, I have one bag of I-don't-know-what (hydrophobic, supposedly "Aerosil 200" which can't be true); one bottle of hydrophilic FS labeled "Aerosil-Cabosil" (which is unlikely)... and 12 large bottles of specifically labeled and verified product that I sourced directly from manufacturers.
B&S is, in this case, tangled in the same problem. I've sent them mail asking *which* fumed silica they're selling, and haven't yet gotten a reply. I wonder if that's because they don't know for certain. It definitely seems to be endemic in the FS reseller trade. I've also recommended that folks buy directly from them; at least buying from them nets you a hydrophilic version.
But as far as I can tell, nobody yet knows what the "right stuff" is. As long as it's hydrophilic it seems to convey some benefit, and that's as far as the scientific analysis has gone; beyond that, each person's research is only valid to their setup, as they have a different FS than everyone else (and may not even know which FS they've got).
I could guess that Aerosil 380 is more likely to increase dmax, and Aerosil 90 more likely to streak. But those are based on other uses of those products, and not based on any evidence.
What I want to do is actually catalog how some/several/all of the variants perform, so that real qualitative comparisons can be made. And if there is indeed some particular "right stuff" that performs better, people will know to look for it. Perhaps B&S would wind up stocking that variant.
So far it sounds like this would be beneficial to those experimenting with FS, and nobody actually knows what they've got, so I might as well continue down the road I'm on...