“Full Impulse” is generally considered to be 0.25c.
The force of an impact of a Voyager-sized (700 kilotons) mass at that speed would be many times greater than that of the asteroid that killed the dinosaurs.
And I think its reasonable to assume that 0.25c isn’t a hard limit, but rather an agreed-upon speed limit for starships.
If you could make an object go even faster, that energy goes up by a few orders of magnitude.
0.9c sounds doable. I don’t know about any faster, but maybe?

At that speed, the force of Voyager hitting a planet would be at least hundreds of times greater than the aforementioned asteroid. This sounds like it would completely sterilize the planet.

Which begs the question, why don’t we see weapons like this in star trek? I’d figure the Federation wouldn’t use them, but the Federation isn’t alone.

One argument I tend to see when this comes up is, that the shields would block it. Which then makes you think, if they could block that, then what couldn’t they block? It makes them pretty much invincible. So I don’t think that’s it.