First off, the Cayuga are nice harvested at about 17-19 brix. You can go higher, but just don't expect to go to far into the 20's which it will. It will begin to lose too much acid and be less tasty in the end.
I had powdery mildew a bit last year and the baking soda dried it right up and it didn't come back. I just didn't think it was plausible to put it on this year to only need to spray again 2 days later. With Elite or something similar, you get forward and backward protection so even though it rained, I was still OK for 10 days to 2 weeks. The spray is more expensive than baking soda, but the time, diesel fuel expenditure and such outweight that little extra cost on a per acre basis.
Yes you can fall prune some to lessen the load later, just don't overdo it. You could easily take extra growth off the trunk.