1) The travel time benefits are degressive owing to increasingly eccentric heliocentric trajectories - the changes in trajectory length get smaller and smaller as your velocity vector stops being colinear with the planet's orbit upon intercept, so you don't really save a lot of additional time. (But you get the most benefits with even small increases above Hohmann transfer speed.)
2) Intercept velocities, on the other hand, are progressive -- pretty much for the same reason, combined with Pythagoras' theorem. At one point you stop being able to aerocapture, even with exerting downward lift in Martian atmosphere to prolong the braking phase.
Owing to these two things, I'm not quite sure that propelling yourself from LEO to Mars at 15 km/s would be a good idea, unless you intend to crash into the planet.
Using a Hohmann transfer orbit [1], you get from Earth to Mars in about 9 months.
Using an Aldrin Mars cycler [2], you can get in as little as 75 days. Of course, the Aldrin Mars cycler requires more delta-v, but that's the point, if you have more delta-v you get there sooner.
This starts becoming disadvantageous even faster than just trying to speed up. At that point a much better improvement for you is the development of a magnetoshell decelerator. Mass-wise, there's no situation where propulsive intercept is better since a magnetoshell decelerator will be much more lightweight than even an NTR stage.
Why? Propellant is relatively light in an NTR system as far as I'm aware. I am also assuming that making a nuclear ship aerodynamic would be unfeasible (it will likely be long and only structurally reinforced longitudinally).
Why is that not mission-to-Mars territory? You can shave months off the transit time with >15km/s delta-v.