If you are confident that the Biltema system is up to the task and that you are able to install it according to every specification the GVK webb page says it should be OK.
On the other hand, how much more will it cost to use a waterproofed sheet material and how much is that of the total cost? Is it "worth it" if you get a water damaged bathroom? If you have to tear down the bathroom and do it again i n a few years because you save that amount now, you will not be happy.
