If those are the facts, then that must mean that you found the definition for augmented maximum in the SR5 book, correct? If so, do tell where it is, as it would help clear up this whole matter.
Now flipping back to SR4A, we can find that it gives its own definition for augmented Attribute maximum:
The standard range of natural human attributes is rated on a scale of 1 to 6, with 3 being average. Physical and Mental attributes have a maximum natural rating of 6 plus or minus metatype modifiers, depending on metatype (p. 81). The maximum augmented attribute value for each metatype is equal to 1.5 times this figure, rounded down (see the Metatype Attribute Table, p. 81). This also applies to Initiative.
In other words:
floor( Natural Attribute Maximum * 1.5 ) = Augmented Attribute Maximum
I do believe this adds to the credibility that at least my formula for augmented Attribute maximum in SR5 is correct, but again, this can't move beyond speculation because SR5 doesn't say so for itself (unless you can cite otherwise).
In addition, while SR4A mainly uses "augmentation" to specifically mean cyberware and bioware, there are several instances in which it says that augmentations can be magical in nature, as well. In particular:
As mentioned in Game Concepts, p. 40, normal attributes range between 1 and 6. A character’s metatype may adjust the maximums higher or lower. Augmentation (either through technology or magic) can allow a character to exceed their metatype maximum to a certain point.
We can infer from this, then, that enhancements provided by cyberware, bioware, magic, drugs, etc. cannot exceed the augmented Attribute maximum. It's important to note, however, that there's no such thing as an augmentation bonus cap in SR4A. A human with an Agility of 1 can cyber, magic, and drug his way up to his augmented maximum of 9.
For SR5, the main question then becomes what, if anything, counts toward the augmentation bonus cap of +4 besides cyberware and bioware. Again, it seems that SR5 only uses "augmentation" to refer to cyberware and bioware. Furthermore, if it was intended for magic and other attribute enhancers to count toward the augmentation bonus cap, you'd think that those spells and adept powers making reference to the augmented maximum would instead say something along the lines of, "This spell/power counts toward the Attribute's augmentation bonus (which is capped at +4)."
Common sense would dictate that anything that improves an attribute augments it and is thus subject to the limit of +4 unless specifically noted as an exception. Therefore, common sense dictates that yes, the drugs and such are limited.
In the real world, yes, "improves" implies "augments." In the context of SR5, however, "augmentation," a term that gets used a lot, appears to refer to a very specific thing: cyberware and bioware. Common sense in our world doesn't necessarily transfer to the Sixth World. For example, our common sense says that magic doesn't exist.
