I can't comment on the performance of the tangent delta formulation but they seem to be excessively pricey for the small increase in performance (according the arbitrary scale FF uses to rate their fins). Also, stiffness is not always better because it affects comfort, too. But I'll defer to those who have actually tried them.
I am a long-time user of the FF Pro model and was generally happy with it, especially the fit, bungie strap, and manueverablily but I wanted to improve my air consumption (the classic FF kick did not seem conducive to that) and I also wanted a fin that I could frog kick with when I wanted (doole, I am truely impressed that you can frog yours).
So, I was asking the support guy at FF about their fins (I kind of wanted to probe someone who has access to them all) and he said his favorite by far was the regular Excellerating model which is longer and narrower than the classic FF shape of the Original and Pro models.
So, based on that recommendaton I got the Excellerating model. I have 18 dives on it an and for me is the perfect fin. I can't think of any thing I would change on it. Slower kick cycle and adequate frog kick. Force Fin detractors should try that model.
KOMPRESSOR, it seems you like your current Pro model but just need it stiffer. Also, you said you weren't very concerned about cost. Seems you would be best with the stiffer tan delta polyurethane in either the classic shape or the slightly larger Extra model.