I had asked the same question at about the same mileage, 4 years ago.
A very noticeable diffirence. You might consider this a break-in period, and yes the 6.0 Ltr engine does get better with hours on the engine.
Now it might be time to get yourself a precision tune, in order to get a little more power and less limitations.
There are quite a few mods available to really open up the LS2 potential.