In my experience with the 135mm and the 1.7x TC, I have been amazed at the performance. This is not unexpected if you look over the MTF curves on the Hasselblad website. I have the H series 1.7x TC, and there is no comparison. I think it was brilliant what Hasselblad did in designing a medium telephoto with a 1.7x TC matched expressly for it, giving you both an exceptional 135mm lens and an exceptional 240mm lens in one relatively compact package. I previously posted some photos that I took with and without the TC.
I think it shows us, that modern designs like the 135mm + 1.7x TC = 230mm lens and Fujis GF 250mm + 1,4 = 350mm, are just stunning and have nothing to do with older Designs von Hassi, Phase, Nikon or Canon. I can only speak from the GFX perspective, however I'm sure after reading only good things about the 135mm + 1.7x TC combination it's equally good. The performance is nearly identical to not using the TC. I had Images from the GF250 @ 350, which were so good, I thought they were shot without the converter.
Hasselblad decision to not sell the converter alone, makes no sense to me. However, I would have bought them together anyway.