With TenkaraUSA's medium and slower action rods, the 12' Iwana and the 13' Ayu, I have fished smaller nymphs several times with and without an indicator or trailing a dry fly. Those rods handle small nymphs like size 14 and 16 bh pheasant tails and the smallest thingamabobber just fine.

With the Iwana, I also fished a fairly heavy nymph - about a size 12 with a huge bh - with no indicator a couple times and had no problems.

With the Iwana, I fished a heavily weighted ( 25 wraps of .025 non lead wire ) size 6 4XL stonefly nymph - ONCE. That rig worked, in that I caught a very nice cuttbow with it, but it was not really castable. The cast, more of a big slow lob, was more laughable than anything. I got the fly in the water, but accuracy was clearly lacking. Considering I was using a thread furled line / leader and 5X tippet, it wasn't all that bad.

I didn't nymph with TenkaraUSA's faster action Yamame the one day that I tried it. But it does seem to me that that rod, with a beefier line / leader and larger tippet, would most likely cast / fish that big stonefly nymph reasonably well. The problem would be with the heavier tippet - if you hung up on something and had to pull free without being able to actually pull on the line rather than the rod, you would possibly break the tip of the rod.

If we have some open water around here the next few days, I'll try to get the Yamame for a day and see how it does with that big stonefly nymph fished off TenkaraUSA's furled line and maybe a floating ( 5 wt running ) line with heavier tippet, likely a 3X fluoro with a very short piece of 5X tippet to protect the rod.

John