OK, I'll play
I think a medium/progressive action throws the longer line the best.
Faster action rods usually have a definitive "kick" point in the upper 1/2 of the blank. I own quite a few and they have many characteristics I really like and I fish them a lot. But I'm usually not fishing a belly longer than 70' with them.
When you get the proper grains of line past the tiptop, the fast-action speyrod will flex to this point and you'll have hit the "sweet spot" of the blank. Unfortunately, less line is often difficult to cast and more line will usually cause the blank to lose efficiency as the rod is flexed past the point of maximum "kick".
Progressive rods (all the Burkheimers, Sage 9150 and 8150, Diawa, Winston DBFs, etc) have a more continual flex. With the longer belly lines, the short casts are accomplished with the tip of the blank and as greater lengths of line are worked out the blank flexes deeper.
Since these rods have a more continuous taper, they have a greater range of line wts they cast effectively.
In addition, the best of these progressive action rods I've cast will deliver power thru the full range of the cast. What I mean is that, with a long belly line, you'll feel the heaviest load at the initial moment the forward cast is initiated. Then the middle section of the blank will add it's impetus, and finally the tip will roll out adding a final boost.
The final challenge, as I see it, is to attempt to merge the lighter faster characteristics of the one with the more progressive continuous power of the other. Many are experimenting with this, and there's at least one innovative designer making amazing strides in this direction.
Happy Casting!
Brian