Are cast metal gear parts really that much sturdier? I can see how they might be in the larger scales, and/or for larger real-life parts...but for the average 1/48th scale or smaller piece? At least plastic has a bit of flexibility. Now, has there been much work in resin casting, with embedded steel wire? This has been done in the figure arena. It provides strength, plus the superior "detail catching" quality that resin provides.