I'll respond since it's a direct question, but I'm really trying to avoid a long discussion on logic and facilitating communications. Don't think this is the proper location for it. I will avoid responding to the editorial comments.
You don't need mathematical rigour for a forum conversation. You just need a bit of backup for your points.
Logic is the "backup" - the structure of the argument. If you don't have it, you don't have a reasonable argument. You're left with editorial comments, opinions, fallacies, and other unsupported statements. Logic shows your support...or lack thereof.
Anyway, you never explained what the actual problem with my opinion is. So...please do that.
I did. Look back at the fallacy. Whether your conclusion is correct or incorrect, your reasoning was fallacious.
Moving on.
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Going back to the powers (attempting to get back on subject), I'd probably set them up to grant mental consequences and / or the ability to clear a mild mental consequence. Granting consequences is within the realm of a stunt and gives you most of the benefits of a stress box plus a few others. In addition to allowing an extra spell, it can add power to spells and, depending on limitations, soak other mental damage.
And, if you really want to add stress as a stunt, the ability to add a mild consequence as a stunt is probably sufficient precedent.
If you prefer to keep them a power and boost them, you might consider adding armor (modeled after Toughness) or consequence clearing (modeled after Recovery).