Without the resistors the audio levels will be much higher. Usually Sena is loud enough, but i encountered some files, mostly podcasts and audiobooks which are recorded too quiet. Increasing loudness by removing resistors is tempting, but it will make Sena's own system beeps and vocal prompts dangerously loud.
If the speakers are left connected and installed in the helmet, some sound from them will leak through even with insulated earbuds. I don't know how it will affect the listening experience.