Jolt head punched, flat head, shark tooth s/s fittings, screwed or screwed and plugged?
150mm Vitex so needs good fastening to prevent any cupping. Has anyone had any experience in this area?

Fixings are very unlikely to prevent cupping because of the forces involved. The strongest fixings would be screws, and these also have the advantage of not popping up later. For reducing the chance of cupping make sure the boards have anti cupping grooves run on the undersides. Other ways to reduce cupping include using quarter sawn timber.