It would not be a direct impact force. That would work more like a nail hitting a piece of wood.
GF seemed to approach this situation logically. Think of the asteroid as a semi-soft clay ball, roughly the size of a soccer ball. If a small, thin object were to connect with it while exerting the energy required to shatter it, the object would easily pierce the ball. Think of pressing a battery into the ball for this example.
Now, imagine a black cat firework, roughly the same size as the battery used in example one. Press this into the clay ball and light the fuse. When the firework explodes, the clay ball will shatter as well. This will be example two.
Example three will use the same firework, only this time, it will be placed against the clay ball. When the firework goes off, the clay ball will move, but we'll likely receive very little damage, if any.
I imagine this playing out like scenario two. If Rayquaza simply flew hard at the asteroid, he would have gone through it. If he hit it with an energy blast from the outside, he would merely have moved it off course or possibly split it.
Rayquaza entered the meteor and unleashed his draconian energy. Having nowhere to go, said energy would build up pressure and, ultimately, cause the meteor to explode dramatically. Now... let's assume the meteor is highly silicon based, as many spatial entities are, as are most non gas planets. Basically, we are looking at a giant rock.
It was not a sphere, so I will eyeball a guess that the rock was 6 miles at its longest point and not its shortest. That gives a maximum radius of 3 miles. Rayquaza had to exert at least enough energy to shatter that from within. Not hard to create an estimate of the force required.
My best guess? Roughly 40 percent of the force of a nuclear warhead would be the most needed to do this. Therefore, Rayquaza exerted that much energy or more.