400m in one dimension leaves a lot of room for variation. 400m x 1m x 1m is a lot less concerning than a 400m diameter sphere.
Edit: I confused radius and diameter in my calculation and revised it. I may have made other mistakes
For sure the depth of penetration would be different for a rod shaped object of equivalent mass compared to a spheroid, the latter would penetrate much less deep. Angle of incidence would be a factor as well as (obviously) the speed of the impacting body. But if it impacted at a higher speed than that the ejecta could get out of the way it would cause an absolutely massive crater. As though you'd exploded an absolutely enormous nuclear bomb deep underground, but not so deep that the explosion would still reach the surface. That would probably be the worst case scenario for an impact like this.