Yes, the LS1 and LS6 have a coolant line running through the throttle body. I did the bypass myself when I was replacing the throttle body with a Shaner polished unit. As for it really making any difference, not that I could really tell.
I've read the same accounts about how this is really for the more northern climates to heat up the throttle body quickly during really cold weather, but you do have a point that maybe in really HOT temps, it may actually help to cool down the throttle body by leaving the coolant lines in place....
All in all, it's a cheap and easy mod, but not sure actually how much good it does for you.