I think you don't see my point there. You are absolutely right in everything you said, but you are talking about the past. Imagine that there would be a hotfix tomorrow that would increase damage of some Assassination talents. Do you think rogues would start switching and doing theorycrafting based on those new numbers? No, because the "race" is already over and so it does not matter. We'd see some posts like "yeah, Assassination got buffed so it's okay to play it", but nobody would actually do some extensive testing, rewrite guides, update modelling tools etc. It's usually those very good players that dedicate their time for this, but at this point of the game, those players are just farming 8/8hc every week or don't play at all. This is what I am talking about and why I believe that we don't have so many people testing and trying current Assassination in BiS gear.
As for your second point, please name some examples where can we actually observe how does one spec compete against another. Thinking about heroic bosses, every single one of them has something that favours one of the specs. Possibly some normal modes might be taken into consideration (Morchok and Zon'ozz), but normal kills records are even more inflated by players overgearing the content and messing around with numbers. So I just don't see this "plenty of actual testing" that is unbiased, but correct my if I overlooked something.