Firebreath does the same dmg at all empowerment just it is adjusted in how it is dispersed per empowerment. The longer you hold the more instant the dmg is. The less you hold it the longer the Dot of the dmg is.
There is an argument with using empowerment one fire breath and then utilizing Catalyze talent ( WHICH INCREASES THE PERIODIC TICKS BY 100% WHEN CHANNEL DISENTERGRATE ON FIRE BREATH TARGET AFFECTED BY THE DOT).
Combine with Shattering Star and Dragon rage you’re going to truck the target and get random procs of crits and a double-tick dot.
In AoE situations take Everburning flame and use empowered lvl 1 empowerment and spamming pyre with Dense Energy Talent and Volatility. Can lead to a mega-long fire DoT with huge AoE.
Making the instant fire breath only feel worthwhile in PvP or fast-paced fight where mobs are less tanky.
To be honest I like it. But! I DO NOT LIKE THAT IT IS DISPELLABLE. I ALSO DON’T LIKE THAT ETERNITY SURGE FEELS LAME!
Here are my thoughts on improving the Evoker for us 
My thoughts on improving Evoker