No they're not, all they do is hold the sub enclose in place. And they're not hard to remove. Fold down the back seats, take out a couple of the plastic clips that hold the rear deck in, and you'll be able to fold down the "carpet" that's on the wall of the trunk. There are two bolts that hold the brackets in and they're a piece of cake to get out.
By the way, this is a great deal for someone. That sub alone is around $100 (looks like a JL 8W0) and shipping this thing isn't cheap or easy. Bump for a good new guy, he just bought a Thunder manifold off me and was a pleasure to deal with. I'm sure he's as good of a seller as he is a buyer.
And this setup is GREAT in the P5. I had it in mine when I owned one, and I loved it. The install is suuuuper clean cuz this sits right under the privacy cover, and it sounds so much better than it does in a sedan cuz the hatch space isn't separated like the trunk of a sedan is.