I'm glad you're asking that question. I was reading their website today and I noticed that they include a rain cover for the seat at no charge, which leads me to believe it's necessary. Cutting up a seat to install a driver's backrest makes me uneasy and a deal breaker if the seat will soak up water. Reading over their documentation it seems to be a concern for them also.
That's interesting. I had to pay additional for my raincover when I ordered my Ultimate seat.
21 years Army (retired)
...been everywhere, seen everything, done almost everything.