Here in the USA our various race sanctioning organizations (SCCA, NHRA, NASCAR, NASA, etc.) specifiy tubing material type, OD, and wall thickness. We generally don't have a big selection to choose from if we want to race on a track somewhere. And given the half-century of experience those same racing organizations have with accidents, roll overs, etc. their recommendations are correct. As someone who builds roll cages in race cars for a living, I would be reluctant to experiment with my customer's lives by trying alternative materials. In the past, Porsche experimented with aluminum tubing (6061T6 2" OD .135 wall) in their roll cage construction on 934s, 935s, and 936s. They found that the cages worked well on the first impact but tended to fail on subsequent hits.