For me, getting the correct head position has made a huge difference in my stroke.
You can try experimenting with raising and lowering your head position while doing the Superman Glide. You'll be able to glide further when the head position is right.
I am very susceptible to over-correcting. If I hear "hang your head", I'm likely to lower my head to far. If I hear that my head position is too low, I'm likely to raise it too much. For me, a drill that imprints the exact right position is invaluable.
Once you have imprinted the correct head position, take care to keep it in that position when you rotate your neck to breathe. My TI coach advised me to look over my shoulder when I breathe--that has been *extremely* helpful. My instinct is to raise my head (i.e. look forward) when I breathe. That slows me down and makes my neck sore.
After a few sessions practicing the proper head position, I reduced my SPL by 2 strokes. I've been working on this for about a month now, and I still have to be very mindful of it. I'm especially tempted to raise my head while pushing off from the wall or lower it when I get tired or lose concentration.