> are highly susceptible to flair
Not that much. Of course, they have more glass-air-boundaries. But the Mutars are not especially prone to flare. You have problems if the sun is in the image. But then you may have problems with and without mutar. Mutars contain some kind of sun shield.
> at least to 5,6
As written, I recommend f/8 for full sharpness at the edges and in the corners.
> exposure compensation necessary= -0,5 EV
I use no compensation at all and could not detect any difference of negativ density of the same subject taken with and without Mutar.
> not pay 1000 USD
That is quite a lot, of course.