Yes, I think in this climate getting 15% or better off a mid-level trim with either the 6.2 or the diesel is pretty good.
Higher trim lines have more margin and there is less demand for the lower trim lines and standard engines, so their discounts will be greater.
You got a great deal on the buyout of your previous truck so you have to look at the total deal.
I got 15.8% off MSRP on my RST Z71 3.0. Could I have haggled for another $500 or $1000? Probably, but I received what I thought were fair trade values on 2 vehicles and that saved me over $2000 in sales tax here in MA. I was very happy with the total deal.
Enjoy your new truck!