Tom,
The R3 comes with a closed cell foam U-shaped pad. It is used in place of a seat cushion if you choose to remove them. Since I have the GT3 seats, I remove the seatback cushion (velcroed in) and velcro in the U-shaped pad in its place. When I sit, the R3 fits in the U-shape and essentially locks me in place with a comfortable, level, back rest.

Even if you chose not to use the U-pad, the R3 is finished Carbon Fiber as you can see in the photos. So, it shouldn't affect the seats in any harmful way.

One more thing. I find that it is far easier to first attach the R3 to the helmet, like in my photos, put the helmet on, attach the R3 straps and THEN get in the car. It adds maybe one minute to the ready time. Not too shabby in my opinion.

After having this and using it, I would literally feel "unsafe" if I contemplate not using it. Funny how that works! LOL!

 

Since I might very well be that student , you'll be happy to know that LFT sells an instructor pad for the R3. Its under the 'Safety Accessories' section of their catalog page.