I wouldn't give too much credence to what Ken Rockwell says, the Mamiya Lenses might not be as good as the latest Carl Zeiss or Mamiya 7 lenses but they are not bad lenses. Using a multicoated filter to overcome flare or coating problems enhances the problem one more glass surface means stronger possible flare and a contrast reduction. A lens hood helps much more against flare than any filter. Also check out if your lens is clean (free of dirt and scratches) and free of fungus both can enhance flare and severely reduce the sharpness of the lens.

Good luck