You can reach that level of damage because the Thunder->Thundara combo increases damage a considerable amount.

Thundaga does high damage when it crits, I usually use excruciate(sp?) Before Thundara and it tends to activate on Thundaga.

Burst achieves even higher damage than Thundaga and costs less MP so long as you meet the low HP requirement, I usually burn my MP down to double didgets using Thunder -> Thundara then Convert -> Excruciate -> Necrogenisis -> Burst. You get the most out of your burst as well as recover a good amount of HP all while having MP back from your convert in one go.

I would say that it's requirements for it's high damage output are acceptable and that it being a 15 minute recast is also acceptable.
I usually follow up a Burst combo with a Thundaga combo, it's a very large amount of damage in a very short period of time.

If you have situational awareness using convert isn't as risky as it seems.