I'm considering an ultralight (non-waterproof) shell and Nikwax wash-in waterproofing as a rain layer. I'd like it to be waterproof through several wearings (a week or more).
Does this result in a garment which will actually repel water in a heavy rain?
I don’t think that will work for heavy rain. The waterproofing coating will make the water slide away from the garment (this is the so-called lotus effect), but that’s just one part of keeping the water out. The harder part is not letting the water through under pressure, like when you press the garment against something, under your backpack straps, under heavy rainfall etc. Here the simple waterproofing finish will eventually give way and the water will get through.
My one experience can be summarised as "ish". It definitely helped / worked in light rain, so if that's all you're trying to achieve then it should be fine. But for heavy rain or pressing the garment against a wet surface, it was all but useless.
Afraid it was a while back and I can't remember the specific brand I used, but after talking to a couple of other people who've also tried it they seemed to have similar experiences.
