I had some difficulty getting to wind in my 17. Mine is a Mark I, meanings the original design with no cabin and no water ballast. Wind was 20-25, estimated, with waves about 2 feet. I try to estimate conservatively. I asked Graham for some advice, and here is a summary of the Old Master's response, with my comments in (parens). I'm trying to summarize accurately, but I invite Graham to jump in if I misstate or omit.
As the breeze freshens, first tighten the snotters to flatten and depower the sails. Then "lower" (assume he means reef) the sails "as much as I can get away with to reduce the heeling force." Then (and this was a surprise to me) "I raise the centerboard so that it rakes to about 45 degrees aft to raise the center of lateral resistance, also reducing the heeling force. "You do need to keep up some speed to make up for the smaller centerboard area."
Then Graham adds that "the worst thing that you can do is to pinch, because the waves will stop you. Sheet a little further out than normal but not too much and try to sail her flat and full. I like a fair amount of weather helm in these conditions. My first reef is just the main. If I still cannot hold her up I will reef the mizzen. If you take away too much power, you will be slow in the lulls. Rather than feather too much in the puffs, you are better to ease the main for a few seconds and bring it back on quickly. If you have speed you can come up for the bigger waves as long as you fall back on course right away." He adds, finally, "sailing at large angles of heel is slow."
I am eager to try all this, but my breezy sail was the last of the season and the boat needed some repairs after that trip and is now put away. That day, I had put in double reefs on both main and mizzen. I'm wondering now if I would have been better off with just a single reef on the mizzen.
Finally, as others have noted: tie the reef points around the sail only, not around the sprits. And the sprits should be on alternate sides, just so that any effects of a "bad side" are balanced.
Note again that this is for the Mark I, not for the water-ballasted Mark III that you have, where results and techniques may vary. But you certainly don't want to be out in a blow with lee helm.
It's interesting to read all the comments. I'm confident the Core Sounds can go upwind in a blow because, among other things, Graham and Alan have done it. I think it just turns out that the groove is a little narrower than on other points of sail.