I just installed a set of Pioneer splits in the front of a VZ - they're also 6" speakers out of the factory, mounted in plastic pods.
The Pioneer jobs (can't remember the model, 6.5" kevlar bass/mid, and a 25mm soft dome tweeter) are a 6.5" speaker, but fit in the pod cutouts. The only hassle is that the magnets are a bit larger diameter, and the tapered "half cone" on the back of the pod has to be cut to clear it. (the driver is no deeper than the original though)
I've mounted other 6.5" speakers though which
wouldn't fit in the pod cutouts, I had to trim about 4mm around the inside edge to get 'em in. In my own humble opinion, the "bit of fiddling around" is more than worth it though - them Response splits are SWEEEEET!
I guess some won't want to "mutilate" the plastic pods, but at least they could be purchased later if you wanted to return it to stock. (the only question I would have is.... WHY??)
For what it's worth, anyhow.
Cheerz!