You don't seem to indicate that anything is wrong with the stock speaker. Unless you are willing and able to do the necessary engineering, leave it alone. If the cab does not suit your needs, sell it and get something else. If you love the cab, but need more SPL, hunt down a matching cab (I do understand this will probably not be easy).
The internal volume of the cab and tuning of the port must be designed to work with the driver. You could spend a lot of money on a driver that is arguably far superior to the stock driver and wind up with far worse performance. Buying a driver with a higher RMS power rating is no guarantee that the driver will actually handle more power in the cab.
The driver's RMS power rating is a measure of how much heat the voice coil can dissipate without burning. With bass cabs, you need to be concerned with the mechanical power handling below 100hz. In a well designed cab the mechanical power handling may be as low as 50-60% of the drivers RMS power rating. If the cab is not designed specifically to work with the driver, it can be even worse.
Additionally, if the cab is not tuned to work well with the driver, the low frequency response may be uneven or weak. Here is how a ported cab works. At higher frequencies the cab performs pretty much like a sealed design. As the frequency goes lower, at some point the driver's output will start to decrease. The port must be tuned so that it reinforces and smoothly extends response to a lower frequency. As the frequency goes even lower, the port will reproduce more and more of the sound. At the tuning frequency (Fb), the port is producing most of the sound and also providing maximum suppression of the driver's cone movement. Below Fb, output drops of quickly and driver excursion increase quickly, so it's a bad idea to feed the cab a lot of power very far below Fb.
