He's right. By the time you tear in there (remove the entire side quarter panel trim) and access the factory enclosure, tear it apart to get the factory sub out and install your new 6" woofer, you mine as well use an 8 or a 10 and build a new box. The factory enclosure is a REAL PITA to get out and apart.
Pyramid woofers suck, I have had them before, I got suckered into them when I was 16 (1989), they were 12" DVC Super Pro's or wahtever and they sucked! They made a ton of bass, but it was not clear, it was distorted, and they didnt last too long....
Youcan buy other off brand woofers that will make the Pyramid look dumb for the same $$$$.
If youare going to use the factory amp to push it you want something with a small magnet (rule of thumb) and a single voice coil with a 4ohm load. Like a good Pioneer 8", or similar. then you can remove the factory POS and cut a larger hole in teh enclosure for the 8", and stuff it full of insulation for deeper bass....