I have to say that having played multiple AE classes, I think that SoC is very powerful and quite balanced.
The jist of it is, you can spam the hell out of this powerful Targeted AE as long you're not taking hits. Realistically its a few milisecond difference between the Cast Time and the GCD, so the downside is obviously that you just cannot move, or get hit.
For fights like Illhoof obviously this makes it difficult to spam (since your getting hit with dozens of Fireballs), but brings up other options which I believe make the game balanced. Being able to cast SoC while on the move without interuption would be simply ridiculous.
As for Empowered Corruption, I forget how low exactly but the +Spell coefficient for SoC explosion is so bad anyways (16%, 6%? -- something) that you're better off speccing for Crit% if you want to boost Seed numbers.