A friend threw me a tip for waterproofing winter gear. It's a product called Nikwax Techwash. Add a couple of caps full into a bucket and wash + rinse + dry. It's recommended by the outdoorsy types who treat their Goretex jackets in it every couple of seasons to make the fabric hydrophobic, so moisture beads on the outside rather than soaking in. A longer lasting and more effective product than spray-on treatments.
All I've done so far is soak a pair of gloves in it.. Testing yet to occur. I'm not sure if I'd go as far as treating knicks and jerseys, but soft shell jackets or vests maybe.
Haven't tried it on cycling gear but have done it on softshell for bushwalking. It works for light showers but once it starts bucketing down there's not much you can do. I've heard it reduces breathability so maybe not such a good idea on those summer jerseys.