I have a gold Variax 700. When a couple of my piezos went (I cant remember which strings), I replaced them with Graphtecs. I did the electronic part (fitting and soldering etc.. ) as I am an aviation electronics technician. Unfortunately, I could not get the intonation set. There just wasn't enough adjustment room on the bridge. I sent the guitar to a tech in London (Guitar Aid) and he had to reset the neck in order to bring the bridge adjustment into range. He did a beautfiul job and the guitar intonation and set up is now perfect. That was a few years ago and, after about 500 hrs of playing it, I have not had a problem since.
I dont know if the new JT Variaxs might have the same issue.
Oh, and the Graphtecs are much more powerful than the original piezos.