Cortland recently partnered with Kelly Galloup, and they are set to introduce newly designed sinking/streamer lines this spring. I would guess just weeks from now. It might be worth waiting to see what they come up with; although I doubt the new lines will revolutionize streamer fishing.
I suggest searching YouTube for some of Kelly's streamer fishing videos. He is one of the most highly regarded streamer fishermen, and he is not shy about letting you know it, lol. Even though he fishes a lot of sinking or sinking head lines, he does not usually fish them deep. He has said on multiple occasions that most of his fish are caught on streamers over two or three feet of water. So, he doesn't use sinking lines to necessarily fish deep.
Often, he can watch his streamer being retrieved just under the surface and see the fish's take. The sinking line provides a direct connection with the leader and fly without creating a hinge effect you can get with a floating line, mono leader and subsurface fly. This gives the fisherman more control over the fly's depth and movement in the water. The moment you start retrieving the fly, the line essentially stops sinking. You can fish it shallow or fish it a bit deeper depending upon when you start your retrieve.
I don't have much experience with sinking lines and streamers, but I've been looking into it, and I hope to incorporate more of it into my fishing.