I have now reached level 6. You do not only get Heart of the Storm, but also a handful of thematically fitting spells like Call Lightning, Gust of Wind etc., and that is really great. Spell knowledge is a sorcerer's most valuable commodity, and this means you don't need to cut down on critical spells to get your thunder and lightning-themed magic. I don't know yet how useful those spells are going to be, but it's nice that you have them.
Also, it appears that the extra damage from Heart of the Storm always hits, even if your spell didn't hit its target. And finally, together with Tempestuous Magic you have lots of opportunities for hit-and-run type spell attacks. Get close enough for the extra damage to have targets, cast your attack spell, then use your flight ability to move out of the danger zone. I'm not sure how all this compares in power, but it's great to play.