If you're not only concerned about bugs but also about people inserting backdoors into your hardware it's not enough to have access to the architecture design,

If you're not only concerned about bugs but also about people inserting backdoors into your hardware it's not enough to have access to the architecture design, you want to be able to publicly audit and review the entire manufacturing process (to make sure that the design you audited is actually the one etched on the silicon). I doubt Intel & friends would open their high-end fabs to public scrutiny so I think you'd have to use older technology available "in the open". For this reason alone I don't think a truly open processor could compete directly performance-wise with the current high-end CPUs regardless of architectural choices.