[JJ_SO]

Noise is given - more or less, but always visible in dark sitiations, Fooling around with 1/3 or less stops is only theoretically relevant. In reality, there's just much more and much better reasons to buy into fast or even expensive glass (if one wants to afford it), such as

• making the AF's work easier (although at the same time fast glass = big and heavy glass = slower AF movement)
  
• getting a nicer bokeh, abetter 3D-pop
  
• getting a probbaly (but not always) better build quality because these lenses are expected to be more costly and they have to deliver more precise and reliable movements (which doesn't mean, all fast lenses do so).
  

T-stops I saw mostly on cine-lenses. I can't remember to have seen a cine lens tested from PZ. Lensrentals might be a better address for them. They just recnetl tested some on their optical bench.