Richard, I have both the 180mm (CF) and 250mm (CF lenses - both are absolutely sensational. I use both for landscape and portrait work.
The 180mm is renowned as one of Zeiss' sharpest lenses ever. Some say it is too sharp for portraits as it will pick up so much detail (I use Softar 1 so ladies can compare "before and after"). The 250mm is generally agreed to be just as good and I concurr. Remember that the 180 is one of Zeiss' few new lens designs - not introduced all that long ago.
Having 80mm and 50mm (FLE), I selected the 180mm for portraits and for compressed landscapes over the 150mm - more impact and tighter frame than 150 for me and its reputation for being sharper than sharp convinced me. It is a thing of beauty, I can't say enough about how good it is. It delivers impact, detail, beautiful colour, sharpness - every characteristic is superb. It is a truly great lens.
One day I might fill the gap between 80 and 180mm (maybe the 120mm) but it is a gap I am very happy to live with.
If you like the qualities of your 250mm, you will love this. More experienced users than me comment on how similar the attributes are.
No, I do not have or use an extender. But, if you are worried about softening images on the 180 + 2x, and have no issue with the 250 + 2x, you will have no issue with the 180mm (I even shot with it and a 32mm tube in lowish light hand held at 1/30 sec and produced a beautiful B&W flower with a touch of softness that made the shot).
By the way, some people comment that they took the 150mm over the 180mm due to weight and size difference (marginal and I mostly use a tripod anyway). I say who cares!!! Boy are they missing out.
Go for 180mm and fire away.