I've put Nikwax on a windstopper softshell and it really helped for short term protection. I was out the first day in a downpour for 45 minutes to an hour and stayed dry and comfy. At some point I'm sure it will wet through but I don't know how long it will take.

For me Nikwax wears off pretty fast and needs to be re-applied often, but that seems to be true for all the waterproofing products at some point.