Ummmm, if wood truly has effect on the tone of the guitar -as I believe it does- then a difference in the pickguard's thickness or rigidness might eventually have some effect on it too. I wasn't going to pay attention to that, but now I think I am going to listen closely if there really is something to that when I get to change it.
Huge difference indeed. I put an anodized aluminum guard on my Strat back in the 70s and it gave the guitar a much harsher sound. This was a heavy ash Strat, fairly bright-voiced to begin with, running through a Music Man HD130. Not a lot of breakup to be had except at arena-gig volume.*There is. I put a second aluminum guard under my plastic guard with some pups that were emf sensitive (microphonic) hum-dingers and it makes a huge difference.
I actually gigged the guitar that way for a month or so but the other guitarist in the band noticed the brighter tone too. Eventually when I put the factory guard back on, it instantly had its original sound again.
*When cranked, that was actually a great sounding amp. But I could only crank it in the basement, not with the band. It had a master volume but the sold state preamp distortion wasn't great; needed power tube breakup to sweeten it up..