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That's assuming recommended enclosure size which is relatively unchanged from lower to higher RMS subs for the most part. Port area is the main thing that should change with power.thats if given the proper enclosure size. OP is stuck in a small trunk. Smaller boxes will indeed impact output and need more power to reach proper potential.
Motor force, MMS, and compliance determine efficiency. If the motor force is high, it overcomes the latter two. Motor force on larger RMS subwoofers tends to be higher, to a certain point. It's all dependent on the driver.