Hi Pete -
Hope you are well.
This is really dependent on the rod make and model. Some rods single ratings are high and some are low. Some dual ratings are meant for the lines they might be matched with even though none of them would be a DT, for instance a saltwater overhead two-hander. A fraction of one percent of anglers might fish a DT on the beach but it's doubtful that the rod maker would rate for it.
I would be wary of blanket assessments like this, when it comes to rods and lines one size does not fit all nor does one rule of thumb. Cast it for yourself, talk to the pro staff at fly shops, and ask about specific makes and models on the web for good advice from those in the know.
IFFF Certified THCI @ 2005
Capeflyfisher Guide Service
Island Hopper, Guitarist, Incurable Dreamer
and Founder, Worldwide Flyfishing Forum