Except for maybe the NikWax spray-on intended for coated nylon (I think it is called tent and gear proof), none of their products will make a Windstopper soft shell waterproof. Even if there were some miracle spray that would achieve this goal, the garment would still leak like the proverbial sieve because the seams aren't taped.

And yes, you could make your jacket totally waterproof using home methods, but then you wouldn't have a breathable jacket any more after you finished with the seam sealing and the fabric coating.