Higher energy transfers can widen the launch windows, but their frequency remains unchanged. The frequency is due to the synodic period of Earth and Mars. It doesn't matter how fast you can get to a point in Mars' orbit if Mars doesn't happen to be there when you arrive. Any given trajectory will only work when the planets are in one specific configuration relative to one another; having more delta-V to play with means you can choose from a broader range of possible trajectories.

In the limit, there are hyperbolic trajectories that would basically give you such wide launch windows that you could launch whenever, but you're not doing that with chemical rockets.