The 'Burst' of the Fire TB is:
Lvl 1-3: 300 (300 mana)
Lvl 4: 535 (380 mana)
Lvl 5-6: 935 (1,155 mana)
Lvl 7-9: 1,250 (1,535 mana)
Lvl 10-14: 1,650 (1,615 mana)
Lvl 15-20: 1,850 (1,915 mana)
This isn't including anything with CoF as I don't feel that counts as burst.
The 'Burst' of my hybrid is:
Lvl 1: 300 (300 mana)
Lvl 2-3: 500 (800 mana)
Lvl 4-6: 935 (1,080 mana)
Lvl 7-9: 1,450 (1,360 mana)
Lvl 10-14: 1,850 (1,640 mana)
Lvl 15-20: 2,150 (1,640 mana)
With Deep Freeze:
Lvl 15-20: 3,425 (3615 mana)
The only difference in the burst is that I have to go from Fire to Frost. And honestly, the benefit of RoI being ranged and targetable on the ground (and a LONG range at that) FAR out-weights that downside.
Up until level 10, the hybrid build is better mana wise while at every point being the same if not more damage. At level 10, there is a 25 mana difference.
For fun, I also included the true burst of the Hybrid.
So I fail to see where the Fire TB has better burst. Its just easier to do due to the lack of form switching. BUT if they aren't near you, you only get Fireball where the hybrid can hit with full burst at full range.
Edit: Also, I wanted to point out the other differences other then burst.
Fire TB, on top of that burst gets:
CoF
Fire Aura
Hybrid, on top of that burst gets:
An interrupt/cooldown increase
A stun/slow that works on buildings.