Smashing an asteroid into an object the size of a gate would not release enough energy to super nova a solar system, especially not since the asteroid wasn't travelling all that fast to begin with.
Ergo, Latin, the energy came from the gate being shattered and losing containment. So it doesn't matter HOW that gate is shattered and containment is lost, the energy is still released. The codex in ME3 itself basically says so.
Now, I've seen tons of people say "Well, the gates sent the energy forward so it was diverted away from the solar system itself."
Ignoring the fact that energy has no mass, and therefore logically could not be affected by Mass Effect travel (which is also why the explosion overtaking Joker when the Normandy is going FTL is absolute crap), even if that were true, the energy would continue to build until it reached the final gate, then it has to be released. And then you're talking about enough pure radiation that it would sterilize a quarter of the galaxy of all life. A single super nova is already a threat to every living things within thousands of light years around it. Now imagine several hundreds / thousands of super novas (no idea how many gates there are) all concentrated in one place.
Whoever lived behind that final gate is having a Seriously Bad Day if that energy transferrence theory were true.