Ive used similar stuff, and honestly dont think it will make a very good floor. The 4x8 sheets really sag a lot. I used similar sheets to build a huge composter at my work. It was 3/4" thick, and sagged quite a bit over a 16" span in the sun. Sure, it will never rot, but I dont think its great for a floor. It weighs a ton too BTW.
The stuff I used came from Taylors Plastics. Google them and compare, I didnt fully read your link. Starboard is great stuff for many other uses, but im not sure it would make a good floor.
Id consider using the "plastic" deck boards, they are tough, but still really heavy.
S.