Different boat, but I've got a pair of those rail mount holders up on my bow. I can't really say they cause any visibility problems for me, but I also don't have a lower helm, so my eyes are 2 - 3 feet above the top of the holders. You can see them in these pictures. First picture is the view from the helm at low speed, the second is the view up on plane. Both were taken at approximately my eye height while seated.
Fenders in the holders in the pictures are the 10x26 Taylor Made Big B units, which are my normal docking fenders. For travel involving locks, I've got a set of ball fenders that come along (but are otherwise deflated and stored at home). Those don't fit in holders, so they go on the back deck on the inboard side of the hard taff curtains lashed up tight to the railing.