I have always gone with 1/2 overlap using 2 inch wide heat wrap. This gives a double layer of wrap which IMO is plenty. Any more I think is just extra weight. The copper and the titanium wraps are very effective.
My goals are twofold, on the turbo, header, and uppipe it's more to keep the heat in the pipe and out of the engine compartment, on the DP it was only to keep the heat out of the engine compartment.