My thinking is that if you have a rock headed your way at say 10% of lightspeed or so, pretty much anything you can do to it past a certain point is mostly just going to change what form of matter smacks into your planet.

Maybe my mental model for how that works is wrong, but I'm just picturing a dust/rubble cloud at 10% of c irradiating the planet with xrays or something due to whatever extreme friction interaction happens in the atmosphere.