I have used mine only a couple times -- with the 28" cell behind the shutter and a weak orange (actually a warming filter) filter in front of the shutter. Both times I had the lens closed down to F45 or f64.
The negatives are a little soft in the corners. I would have to do more testing to see if it was truely the lens at fault or if it was the operator. My Zone VI is at its bellows limit to focus at infinity with the 28". Perhaps having the lens closed down all the way did the lens an injustice.
If I had an 8x10 back on my Kodak 2D, I would be able to play with the lens a little more, as it has enough bellows. I have never used the TR in the 12" and 21" because I have better lenses for those focal lengths, and there is some of separation in the 21" element.
Below is an image taken with the 28" element (neg scan).