3 ways to get more oomph from your door speakers:
- Increase sensitivity! - Use a speaker with lightweight soft part (paper, foam, cloth, etc).
- Magnetic field strength or length of wire in the voice coil gap.
- Increase the power!
- Decrease the effective frequency range of the speaker.
Keep in mind that your speakers need to be water/humidity-resistant, dust-resistant, corrosion-resistant, and built sturdy enough to survive ~100,000 door slam events (20 - 50 G mechanical shock).
My Dayton RS180-4 door speakers add some low-end extension and have comparable SPL output... but they're much heavier. They add alot of magnet, lower the soft parts weight, reduce the power, and are likely to accumulate dust due to the phase plug. It also required a complicated adapter to fit in the door.