Lowering the tank and regulator down on your back (while you are standing up) is useful because in the swimming position most divers are never truly perfectly horizontal. Most divers tend to be at a slope of about 10 degrees (you can see in most pictures).
Even technical divers think they are perfectly horizontal all the time, but they are not. To look forward if the diver was in a perfectly horizontal position, the neck would have to bend up a true 90 degrees. It would be a true pain in the neck to try to hold even close to that position for any significant amount of time.
Therefore, lowering the tank to place the regulator between your shoulder blades helps. But, far more important is making sure the regulator itself is close to the divers back and not separated by a thick BC, a big diameter cylinder, or the tank floating up away from the diver. Those will affect performance directly.
Every inch the cylinder (and regulator) moves up away from the diver is an extra inch of water column the diver has to suck to get air. It is a one to one relation (in the vertical direction, in the water column).
If you need to use big diameter cylinders, there is a solution, but at first it may sound a bit crazy. You can add the DIN fitting adapter to the Argonaut and then even use a DIN to yoke adapter. This will extend the regulator a lot. It should actually extend a bit past the plane of the backpack (or back-plate). You would have to use a double-hose back-plate or a backpack that is either flexible or has clearance for the regulator can.
With that extension the regulator should actually touch your back (it is very comfortable even with a very thin wetsuit). I specifically designed the regulator can with rounded soft edges so that it was comfortable if it touches the divers back.
In that position the regulator will breathe at its best.
The only way to make a regulator breath better is to have it surgically implanted.
I am not recommending that.
Having a buddy take some pictures of you (like Mark is suggesting) can be very helpful. Notice that I use plural as in several pictures. Depending on the angle a picture is taken, it can actually be very misleading due to parallax error. The easiest way to solve that is to have him or her take several pictures from a few different angles.
And most important, have fun experimenting and diving.