Adam,

You asked the following question:

What's the highest feasible capture speed then for Mars?

Your question is more complicated than you realize. The first issue is whether or not the vehicle is manned. The maximum allowable decelleration for a manned Mars vehicle is 4 gravities (this assumes the crew was in zero G during transit from Earth to Mars). To aerocapture into Mars orbit and stay under 4 gravities means the vehicle must fly a lift-down trajectory. By flying lift-down, the vehicle can stay in the atmosphere a long time where the aerodynamic lift is keeping the vehicle in the atmosphere while centripetal acceleration is trying to fling the vehicle out of the atmosphere. This combination causes the vehicle to fly at roughly constant altitude for a considerable distance while blowing off kinetic energy (and building up a significant heat load) in the atmosphere. Eventually the centripetal acceleration wins out over lift and the vehicle is flung out onto it's aerocapture orbit. If the Martian atmosphere is assumed to match the COSPAR-Northern/Summer model then the maximum entry velocity is 11 km/sec inertial at an entry altitude of 130.819 km. There is a gotcha with this and that is the vehicle must be maneuvered by roll modulation to stay within its entry corridor. It can't fly at a fix lift-up trajectory because it's too sensitive to variations in a real atmosphere and errors in the entry vector.

If the vehicle is unmanned then all bets are off. What limits aerocapture in this case is minimum altitude accuracy. One finds that to aerocapture for high speed entry with no decelleration limits then there are situations where the vehicle is screaming along at Mach 20 at an altitude of 200 meters. Obviously it would be impossible to actually fly something going that fast that low. Also the thermal protection requirements would be near impossible. This is an example of the weird situation where the trajectory is theoretically possible but the vehicle technologically impossible to actually manufacture.

Gary