I would not pay 1000 USD for this kind of thing! Never!
Here are the specs from Walther Heerings Great Rolleiflex Book:
Mutar 1,5x with Rolleiflex 80/2,8: focal length=116mm (not that much of an improvement IMO), angle of view= 38°, minimum focusing distance= 1,86m, exposure compensation necessary= -0,5 EV
It is further stated that one should stop down at least to 5,6 to get full sharpness and that the Mutars are highly susceptible to flair and should therefore be used well shaded.