You're not going to find Shimano current square taper bottom brackets classified by component level any more. I believe the Tiagra ST crank calls for a UN-53, but Shimano only makes 2 levels, cheap and better. UN25 is cheap, the UN54 (replaced to UN53) is better and the UN55 is more heavy duty. There were some pretty high end Shimano BB in the past, such as the UN-90/91 and UN-70/71 (Dura Ace/XTR and Ultegra/XT) but as you said they were meant for light weight and some say didn't last as long as UN51/52 at the time. So really calling a modern day Shimano square taper BB 105 or Dura Ace is really meaningless because there is only 3 to chose from and the upper 2 are almost identical. You have to go to the exterior BB or oversize press to fit before you start seeing component levels that mean anything.