You're not misunderstanding the way the SVP should work, however I can't say I've ever noticed an increase in signal strength when moving from what should be the shadow zone into the sound channel. (I think LW disagrees with me on that, though). All I can say for sure is that the number of sound paths modeled is rather limited.
In any case, you still have a significant detection range advantage over the skimmers, and the layer will help you get in rather close undetected at low to moderate speeds. You also have the option of putting a UUV up top, although that's going to clutter up your sonar a lot and/or slow down your aTMA (and take up a useful torp tube).
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