Before you rewire, post a pick of how it's wired now. It SHOULD already be wired as a 4ohm box if it's a mass-produced item. Mfgrs sell stuff standardized and then let us experimenters mess around. 4ohm is the car audio standard.
If you are running 4ohms and the amp is ~300 watts at 4ohms, it should hit quite nicely. Even if it's actually only a 200watt amp at 4ohms it will hit about 4dB louder at max than your old amp.
It takes 2x the power to make just a 3dB gain in volume.
It takes 10x the power to double the volume.