...the idea that anyone could tell the difference between a glass flex M5 and a performance core M5 just by surfing "blind" does raise some questions for sure you'd have to be on the same board all day on perfect waves (imo) to notice the swap and only if you are a pro surfer.
I can't agree with you on that one, brother. Materials, construction, and design (template, foil, etc...) all play a significant role in fin performance. I'm no pro, that's for sure... but I can certainly tell the difference between glass flex and PC. In fact, at one point I was so convinced that one out performed the other that I spent a nice chunk of change switching over. For most shortboards, PCs are my choice of construction.