For SWIM, a breakthrough is generally characterized as dissociation or total immersion by psychedelia. Though sometimes it is relative to the particular substance. SWIM has definitely broken through with DMT and Psilocybe, but a mescaline breakthrough is a little less intense than the former two but drastically more rich than sub-breakthrough (whereas, it can be a bit more gradual with DMT and Psilocybe).

I hesitate to use the term now due to it's ambiguity. I'd say it's possible to be fully "in it" as in losing awareness/memory of body/face/breathing, but I would call that the upper limit of sub-breakthrough. There is another level, probably many more. After each time I see deeper, I hear on the way out "yes, yes you can go farther"