The gravitational pull on such and object would only differ between Mars and Earth very close up and for an object moving at 20k/s, that is not many seconds to accelerate it. There is also the point that Earth has a very dense atmosphere which would actually tend to slow down such a body (as well as heating it up) while Mars has a very thin atmosphere. Also as I have pointed out before, with just a third of the gravity, material ejected into the air will go far higher and stay in the air far longer. Mars has a density of 3.9 compared to Earth's 5.5 (that of water), allowing that Mars is also believed to have an iron core. The 23% difference will be cancelled by the softer surface of Mars, allowing for more damage by an impact.