The Rechannel Talent Reads:
When Tranquility is active, Sienna’s ranged charge time is reduced by 40%.
It is also reducing the amount of heat generated to fully charge, and that is particularly beneficial for (and most noticeable on) the Flamewave Staff. Either the error is in the effect, or the text on the talent is.
I didn’t know if there were any mods to see this effect numerically, but these screenshot comparisons show it clearly enough. I was using Voracious Conjuring for T1 talent for all tests, and Heat Sink (20% less heat generated) on both staffs, because I’m lazy.
Fully charged vs finished firing, with and without Rechannel on Conflag and Flamewave:
Clearly it is impacting the heat generated while charging, and since it is the only staff which generates the bulk of it’s charged attack heat before firing, this is most noticeable on Flamewave.