That's true, but having tested out both seats on my bike, I find the GT much more comfortable, and i'm quite tall at 6' 3".
I guess that we are all built a bit differently to each other, so we have to try these things out for our own preferences.
You are also spot on regarding how the stock seats can dig in at the back and cause discomfort over a longer journey. Perhaps the Corbin is better in all aspects??