No disrespect for BOB or Lefty BUT this data and line weight vs: "stated" rod weight and all the "standards" created by the tackle manufacturers were designed for the "average flyfisher".
Funny thing.... long before Lefty wrote the yellow book, Longer Fly Casting, my friends and me were already doing it in Maine to reach Stripers from the beach. The technique is valid but not really needed for trout fishing with a five weight unless you have no wading boots.
With an 8 or 9 weight I have throw enormous heavy & bulky streamers and admit it's a chore unless the rod has a lot of backbone but this doesn't mean it must be a fast tip action. Lefty's whole body casting stroke lends itself well to a full flex rod in many ways..... I've seen him "show off" and was willing to bet he too was underlining.
My personal observations are that too many rod makers (particularly Sage) underline their rods due to a perception that longer casts are what everyone wants!
I do most of my tailwater fishing with a 2 weight and I throw everything with it from big beads to streamers.... But then again I am the exception to the "rule". LOL but true.
Spose I'm more of a trout hunter; there is less line splash with the 2 weight so I can virtually "line" trout feeding directly up-stream at a very acute angle OR only put my leader over them.
How many of us get a decent drift at 70 feet much less respond well to the take of a dry at that distance?