"I read about how non water resistant it is, particularly the lighter variant, is that 1433?, which you can quite literally see light through the holes if I remember right"
You clearly havent used tyvek 1443r. It is plenty water proof. especially if just being used as a ground cloth.
"1.5 oz for polycryo, 5.5 for tyvek regular housewrap, you're right, not quite 4x heavier, only 3.6x?."
Tyvek 1443r would not weigh 5.5 If it did you would have 13.2 square feet. So is that the size you are comparing? Again tyvek 1443r weighs 1.25 oz per sq/yd
I think if we are going to compare something we should be more specific. Apparently this thread is only referring to "Homewrap tyvek" not tyvek as a whole.
Lol I dont even use a ground sheet so mine is lighter than your polycro.