When we had our ducted heat pump installed, we also had the ducts in the attic covered with extra insulation, as well as spray foam at the top of the foundation to seal that completely. This all really helped.
The latter was a surprising (to me) source of heat leakage. As part of the whole effort we had the house examined in detail for heat and air leakage, including using IR imaging and one of those things with a fan that replaces an exterior door to change the internal pressure to find/quantify air leaks.
