I shot film for many years with my Hasselblad. Since I had the 120 Makro Plannar, I thought I wanted the 250 for the length, but when I tried the 180, I was hooked...
However, I was still a bit "short" on my long lenses. So next I tried a Hasselblad 2xe converter. Worked like a charm and I could never see problems with the images.
Both the Mutar and the 2xe converters are sweat!
What I ended up using the the field was my 50 CF, 180 CF and then using my 2xe to add to the range. Though the lenses were now f/8 and slow, on a tripod even the 50 CF image quality at 100mm was more than adequate. Same for the 180 with the 2xe.
Using the 2xe and my lenses, I had a full-range of focal lengths from 50mm to 360mm and with plenty of image quality to boot! The main thing is to religiously use a tripod, cable release, and great technique so as not to degrade the image quality with vibration since I found that that was the main cause of image softness using the TC.
Later, I added the 1.4 PC-Mutar after getting a 40mm CF and had similar good-quality results, even on the longer lenses, though with them, I seldom used the shift. The 1.4 even worked well with both the 120 and 180mm!
If you do go for a teleconverter, don't scrimp! Get one of the Hasselblad converters and you won't be sorry plus the added benefit of using it with your other lenses.
However, if you find a great price on a CF lens, don't hesitate to latch on to it!