What kind of kinetic weapons are we talking here? Building destroying? City-sized crater-making? Or crust-puncturing-apocalyptic?
A "silver bullet" style round, styled after the commonly used anti-tank armor-penetrating rounds we see today, would certainly penetrate the atmosphere and hit the ground. Damaging things if it hit them of course. This is how i'd imagine railguns to work on planets. *plink plink plink* I wouldn't expect them to be effective for saturation bombing.
Why not just allow an offensive mass driver? Mass packets penetrate the atmosphere with no problem, killing millions and destroying infrastructure-- and those aren't even guided. Talk about low-cost weapons of interstellar terror.
A crust-puncturing type kinetic impact is a fairly low-tech orbital bombardment solution as well. Find a suitable orbiting 'roid, hook some rockets to it, and sling it into pretty-much anywhere. Make a whole bunch of poisonous gas, depending on the tectonics and internal structure of the planet. Good thing we can't move roids currently, because that would be *fun*