Those screws are going into an insert. Depending on what maintenance has been done in the past during any rebedding, there can be a substantial amount of sealer in the threads. The last time I disassembled one, the screws came out, but slowly, and with considerable force. It was all the gunk on the screws. Actually, there was a lot of resistance due to the bedding gunk between the screws and the actual deck organizer once removed!
I personally would be intimidated using an impact driver. I cannot recall the details of the inserts, but I would not want to spin one, if the screw is only boud up in the insert and not the organizer. Rather, I would try to slowly back them out. Find an appropriate tipped screw driver that can completely engage the Phillips head, and also has a hex-drive feature below the handle. Many larger mechanics screw drivers can be turned with a wrench for extra leverage.
Bear down as hard as you can to prevent the screw driver from camming out, and use a wrench to increase your lever effort. This hopefully wll get the screws to yield.