Oh, I am sure it can do alot more technology and size wise. Never doubted that. It's just that the shape does not lend itself to much control aerodynamically. It's a purely ballistic shape.
First, these landers really aren't supposed to perform like F-22 Raptors. They are primarily a way for the Polaris
crew to get down to a planet's surface and back.
In atmosphere, I imagine they would operate in a manner similar to a helicopter or a VTOL, with vectored thrust from the main engines and smaller thrusters giving them more than adequate maneuverability. Again, you might not want to engage in a dogfight with one, but it should get the job done otherwise.