Maybe it matters what kind of plastic?
I dunno.. I bought a pretty large bag of plastic saddles, back when for cheap, and when I'm trying different things, I take the stock one out, label it, and put it in a drawer, and then make a new saddle from the plastic ones I got. It doesn't seem to change the sound much. I will say the cheap plastic ones I got are pretty hard. Like.. they make a tinkle sound if you drop them on a table.
And like.. nubone is plastic, but is 'better" plastic for whatever reason.
I could be wrong on this, as my experience is somewhat limited, but I've had very little difference from switching from bone to plastic. But again, I'm not sure all plastic is the same.
I bought a kealoha plastic concert for a friend that was on the fence about learning. That came with a saddle made out of some kinda soft white stuff I think was abs. Being what it is.. a saddle change on it.. isn't going to sound alot different.