The term "high performance" is fairly abstract.

I do believe that the most undesirable trait on a kayak is lack of stability, especially for someone who is a casual pdaddler. Many dedicated paddlers put up with it as a trade for speed. Going from a Ches 14 to 19" wide boat is OK if the padder is totally happy. Since you are building and not buying, I would see if she could try what you build somewhere.

You can put skegs and rudders on boats but trying to create stability is not there. People sometimes feel that adding weight is fine for that, but that's the backward approach.

Good luck on your search.

Thre PADDLER is the one who has to like it.