Revised to 18/38/15 after noticing that Empowered Fire does affect frostfire after all:

http://www.wowhead.com/?talent=of0i0...ohduVuohZMIcco

Also note that the bonus 42% functions after spell damage etc is applied, so you could look at it as 42% more of your spell damage + an average 327.6 from the base damage at rank 2.

I'm not sure how this interacts with Empowered Fire, but I believe damage multipliers act on the final calculated value, so it would also be amplified. This means Empowered Fire contributes 21.3% of your spell damage instead, and is absolutely worth taking. This does leave the build with no -threat beyond popping invisibility, so the DKs ability to tank comes into question.

If you prefer, it makes Frostfire Bolt receive 143% of your +dmg with Empowered Fire (1.42 * (3/3.5 + .15)), as well as the average 327.6 from base (at that coefficient, equal to another 229.1 +dmg).

Since Frostfire Bolt itself snares, the original DK tank isn't precisely necessary (FFB being single-target), but you still get the +25% damage for IAE spam.