This paper considers the electromagnetic and structural co-design of synchronous reluctance (SyR) machines for high-speed applications, with the aid of optimization algorithms. The aim of this work is twofold. First, three commercial electrical steel grades are compared to establish quantitatively how steel properties influence the performance of high-speed SyR machines. The considered steel grades are two cobalt-iron alloys and a high-grade non-oriented silicon steel. Second, an original design method is proposed, targeting maximum power density. The results of the analysis show that for the considered machine output power grows with rotational speed up to 70 krpm. Over such speed limit, the performance degrades due to structural limitations. The proposed design procedure is implemented using open source software.