I have used the 1.7x on my 210 and 100mm lenses with the H3DII-31 and was quite pleased with the results. I cannot comment what one might get on the edges with full frame film or with the less cropped H3D-39 sensor but in my opinion the converter was better than any I have used with 35mm. I thought there was very little image degradation in terms of sharpness. Perhaps you have a bad copy. Nonetheless, one problem I did find was that with the 210mm mounted on a tripod it was a bit more tricky to get camera shake free images. The 210mm is a long narrow lens and there is no lens tripod ring. So with the adapter it gets even longer and more subject to vibration. Thus even with mirror lockup, I find you need to be very careful to avoid any movement. Thus my percent of perfectly focused shots is a bit reduced. As long as I use a tripod and mirror lockup, I found the 100mm +1.7x to be as good as the 150mm HC lens in terms of sharpness and image quality. Of course the 300mm, and 300mm +1.7x gives even more reach. I am not using the 300mm very often so going to sell it. Nonetheless, I find the adapter works quite well and very pleased with the results.
I used to keep some example comparisons but finally deleted them. I could redo a few this weekend.
It seems you did a more careful testing than I did. Nevertheless I found the center sharp with the 1.7x but to the borders of the frame (on H3DII-31) it started to get a softness I was not used to see
with my Hasselblad lenses and I decided against the 1.7x. I did the shooting hand holding but leaning on a wall. It was a bright day and the speed was in the safe range for me. The -31 was choosen
over the -39 nearly solely because of it's one stop higher ISO, making it better for hand holding shots.
Maybe I have to work on my sharpness perception but I had very good experiences with shooting the H3DII on a tripod. I have a Gitzo carbon fiber tripod and Arca Swiss ballhead and exposed even until the maximum of 32 seconds and got sharp photos. Of course I use a longer shutter delay than default or mirror lock up.