That was my point earlier. An actual dinosaur killer size asteroid needs a lot of energy to get enough delta-V to accelerate it out of the Asteroid belt or trojan orbits. Lugging something big enough to do that insystem, and then doing the burn isn't going to be subtle. Even a burn to get it into a minimal energy orbit to intersect with your target planet is going to be pretty hard to overlook.
The minimal estimated size for the Chixulub impactor is about 2x10**12 tons. A minimal Jupiter-Earth transfer is about 3.1km/sec worth of delta-V, and that's an orbit that would take years to make the rendezvous with Earth. Even with a 100% efficient reaction drive, that's about 1.5x10**22J of energy, or equivalent to the explosive yield of 3.5x10**6 megatons that you have to somehow conceal while you accelerate the asteroid. Bearing in mind that at a relative velocity of 3km/sec the kinetic energy of an object is roughly equivalent to the explosive yield of its weight in TNT; attempting to sink even 1% waste energy into the object itself is going to make it quite warm.
A city killer could be a lot smaller, but that's some real precision sharpshooting to hit a city-sized target from Jupiter.
Having said this, although I think we now have a sense of the scale of the effort involved in humping dinosaur killers around a star system, we are also in the business of trying to apply real-world physics to a fantastic setting ...
I wouldn't worry about physics touch but would put that up there in terms of sci fi power. Not ringworlds or dyson level power but probably beyond Star Wars and Trek.
Using small asteroids accelerated a'la Mass Effect better idea.