It's useful to have the IWarp filter, interactively smooshing parts of a layer around. But often I need to see the layers underneath it while I'm doing that, so I can check that my distortions look right in front of their background. Since the standard IWarp preview doesn't show background layers, I have to do it by trial and error, attempting many IWarps and keeping the one that works out best. There has to be a better way!
If you're willing to be a little adventurous, you could try the development version, GIMP 2.9.
The old IWarp plugin has been replaced with an on-canvas Warp Transform tool which works in much the same way, but with a full on-canvas preview. And you can turn-down the opacity of the layer you're warping to allow lower layer(s) to show through, or even change the layer's blend mode (to eg. Overlay or Grain merge).