Much has been made of how the reentry burns of Falcon 9 first stages occur at an altitude where the atmosphere is similar in density to the atmosphere of Mars, and so they demonstrate that supersonic retropropulsion will work for Mars entry by the Red Dragon.
However the Falcon 9 first stage slows from around 1.5 km/s to some velocity during that burn, but not to zero. If the stage is returning to Cape Canaveral, it slows to zero and then speeds up in the opposite direction, but that isn't really the same thing. It isn't spending time at low velocities in that atmosphere as a lander would coming in on Mars. As there is no GPS or any beacons on the ground, a lander will spend some time at low speeds making sure it knows exactly how far away the ground is. Plus it would enter the atmosphere at about 3.5 km/s. It would let drag slow it as much as possible but then would it be firing its engines at a speed similar to the F9 1st stage?
Red Dragon is a pretty different shape and mass than an F9 1st stage, and according to this presentation it will incorporate lift to come in pretty level, so it will have a different angle of attack. So, how is information from F9 retropropulsion helping?
My conclusion from this information in this answer by Mark Adler is that control of such a vehicle is challenging and not the same when parameters are changed, which is why I ask:
Once in the supersonic flow, the aerodynamic effects of the messy business end of the rocket can be complicated, making control a bit of a challenge. You can have counterintuitive effects that redirect flow in unexpected directions at different angles of attack.
Predicting the effect of the running engines on the drag is a challenge. The thrust plumes tend to reduce the drag, countering in part the intent of firing the engines to increase deceleration. With enough of a thrust to drag ratio, this is not a show stopper, but you need to be able to predict how large the effect is to know if you have enough fuel. This impact on drag also complicates what happens when you gimbal the engine, which is part of the challenge in #4. Again, high thrust to drag ratio can reduce the surprises here.