The picture is a poor example of practice. I don't troll that slowly.
Last year as others have said, temperatures were quite similar throughout depths later in the season, this year however I'm still finding cold currents 30 ft down differing as much 14 degrees, as late as last Sunday.
Using the example of the picture, if you were trolling 2.7, your lures would be almost motionless in the current.
The highest difference I can remember was just over 3 mph (with surface / Simrad), which would make a big difference whatever direction you were moving into/through/or in that current.
I realize the fishery is amazing, but I can honestly say, the boat's catch rate jumped immediately after the installation of the X4D.
My neighbour who fishes with me and others, admitted scepticism at first, but is a firm believer now.
I wouldn't go without one on any future boat I purchased.