Thanks for the feedback, much appreciated.
Just stumbled across this
shallow mount sub on Ebay, from a company called Rockville. {snip}
If I were replacing the factory sub driver and budget were an issue, maybe even if it's not an issue, I'd choose this
driver. Running simulations, its specs are perfect for the OE enclosure. I'm pretty sure, at least with a spacer ring or a bit of rib cutting, the RE's 4.5" depth will work in the stock enclosure.
To make bass, you've got to move air. The specs on the Rockville show ~8mm peak-to-peak, which means only 4mm one way. The RE Audio I linked has 13mm *one way*.
Ignore the power numbers. If you're lucky, the manufacturer spec'd the wattage where the glue on the coil or the coil wire itself starts to melt. More times than not, especially in car audio, the "marketing" guys get involved and make up a number. In reality, a sub driver's power handling is almost always determined by excursion limits before "wire melting" limits. And excursion is controlled by box volume so there can be no single power handling number...it's a sliding scale where the bigger the box, the lower the power handling as excursion increases with box size.
Case in point, the RE driver I suggested. RE specs the driver at 200/400 watts (RMS/max). In reality, per the simulations, in the OE .6 ft^3 enclosure, the driver has enough excursion to handle 600 watts from 55Hz and up, as long as the coil/glue doesn't melt. Below that frequency, excursion limited power handling drops...down to 155 watts @ 30Hz. For perspecitve, the lowest note on a standard 4 string electric bass is 42Hz. A typical kick drum is usually in the 40-50hz range with some dipping into the 30s. Overall, I'd say RE actually did a pretty good job with their power spec.
You can use any amp with any driver. Just like with proper use of the accelerator pedal, once could drive an F1 car in bumper to bumper traffic. Just because an amp is capable of 1,000,000 watts doesn't mean you'll be sending that to the driver all of the time. If you hear the sound start to distort, back off, you've reached either the amp or driver limits. And a driver doesn't "need" a given amount of power. It starts to move once it sees more than 0.0 volts.

Some just move further than others for a given input. Big numbers sell, but a driver that only needs 100 watts to move 10mm will play just as loud as a driver with a super stiff suspension that needs 1,000 watts to move 10mm. Remember what I said in the beginning...bass is about moving air with the cone.