2x Telextender for Hasselblad or Pentax67, any good?

I tried one once for my P67. Quite disappointed. When you can get a clean used 165/2.8 - a superb lens - these days just as affordably, why on earth would you even think of a tele-converter?

Some years ago, I conducted a rather extensive series of tests on teleconverters and I determined to my satisfaction that the best quality teleconverters do not cause a loss of image quality, but in fact, actually improve it slightly.

I recently bought a gray Pentax 2x teleconverter for my Pentax 67. I haven't shot with it yet, but I have mounted it and performed what amounted to a visual dry run with it. It is providing sharp images and, given Pentax's reputation of making extremely sharp lenses, I suspect this 2x will do well.

I snagged a mint P67 300EDIF for a reasonable price, and no combination system will come close to what that amazing lens does by itself. I also bought a Nikon adapter for it mostly just for fun with wildlife, where I want the reach. But all this quibbling is a waste of time if you place something like that atop a ballhead and/or sissy tripod. You might as well be shooting from a waterbed with a bowl of Jello between the tripod and camera itself.