Ok correct me if I'm wrong but aren't these things focal length reducers?
So really they are making your lens wider/faster not your sensor bigger.
If so given the fact that many medium format lenses are slower than 35mm lenses wouldn't it be easier/cheaper to buy a 55mm f2 35mm lens than an 80mm f2.8 medium format?
I kind of think the whole concept is somewhat of a misnomer, it's like saying putting a doubler on makes an Arri 435 an Arri SR3.
Again please correct me if I've misunderstood what focal length reducers actually are doing.