When you say "fast action" do you mead a tip action or stiff rod? For me fast means tip action and stiff is a rod which is rated wrong which feels nicer with a heavier line.
For a single hand TFO BVK 3wt there is a two piece conversion kit handle which makes it a 10ft switch rod but it does not make it stiffer although in this video a 235gr Ambush and a floating tip is cast. I have not tried it with conversion handle but BVK 3wt is very nice single hand rod.
I like tip action single hand rods and full action DH rods. Tip action DH is much more critical to line weight and head length and more demanding to cast as well but when short and shortish (Scandi) shooting head weights and lengths are tuned to personal "taste" it is very easy, light and fun casting. Full action DH rod behaves rationally with variety of lines and for long casts and long heads absolute the best choice.
Tip action SH rod is good for me because hauling has big role in my SH casting. I have cast only few One SH rods and they are great but I can't say anything about One Switch or DH rods.
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