Actually if you get more than about a 15 degree temp split in heating mode you're just causing stratification.
I sat through an ASHRAE seminar that went over this and I didn't believe it until I tried it in my last truck. At 20 degrees out and driving down the highway, I had my heater and a/c running, but I had the mix temp set more cold than hot and was very very toasty. Discharge air temp was kept around 70 and that was plenty.
Another key is increase your HVAC fan speed in heating mode with the lower discharge temps.
