The difference between a T2 energy burst and a T3 burst is 4.5 points of base damage / level. If we look at overall damage a T3 burst will do an average of 23 points of base damage per level, +1 Draconic knowledge you would be typically casting burst at caster level 26, for an average base damage of 598.
A T2 burst with +20 elemental spell power and +1 caster level will do an average base damage of 18.5 per level, typically casting at caster level 27, and a
20% boost due to spell power, for an average base damage of 499.5, modified to 599.4 due to the spell power boost from dragon heritage.
So a T3 energy burst and a T2 energy burst with the three extra points thrown into dragon heritage/knowledge is as close to equal as you can get. Additionally, with dragon heritage/knowledge your Dragon breath is much more powerful as well, making that a clear win in my book.
Energy Vortex requires me to stand next to mobs for an extended period of time -- that is not a place my PM wants to stand

. If I am standing next to a mob, it better be dead soon after.
As for Empyrian Magic, the only damage spells I cast often is Energy Burst, Dragon Breath, Necrotic Ray, and Delayed Blast Fireball (once in a while for clean up). There is no way I would get Empyrian stacks to anything meaningful. Also, if I am going to dedicate a twist slot to boosting spell damage (versus DC's), there is no substitute for Sense Weakness.