They do: they go up to 125% of the listed power (just slightly over 100.) I should probably indicate this somewhere other than a forum post though.
Especially since MaxHP% works the exact opposite way and always uses your current MaxHP%, including bonus/lost MaxHP.
But.. given I don't really want them to work differently, I'm more than half tempted to go ahead and just pump breath weapons to 0-100 Power, but have them use % of SP and not raw (i.e.: Tireless won't help all that much.) But, if I do that, the cooldowns and/or SP costs will have to go up... and there will be a lot of very considerably more deadly low-mid Tower encounters. To say nothing of the horror that will be 100 Power Consume Life and Consume Vigor (the new HP drain and Max-HP breath weapons... and yes, one of them will be available as a new starting element on a relic. )
Then again, I could just edit them to say 0-100 Power, 100 Power at 125 SP. That'd make it clear and keep it sort of consistent (mostly by making SP checks clearly a raw amount and not %, and making MaxHP checks a %.) That's probably easier.. I think I feel more comfortable if 100 Power breath weapons require a passive to get there.
But on the other hand... 100 Power breath weapons might be just what the doctor orders for Dragon summoners.
I'll think on this one some. At the very least: whatever solution I pivot to will be better documented than what we've got currently. Thanks. If nothing else, this thread is good for keeping me honest when I've gotten too cute on various mechanics and/or too sloppy in my documentation.