insulate before you generate !
admittedly i only have an 88 station wagon, but can get it to t-shirt weather in there. overhauled the heater box when i got the landy, new multicolumn matrix in there, fixed all the holes in the box, new foam seals between the matrix and box, new trunking between the blower motor and heater box. its ok now, not modern car good, but ok !
i think half the issue is that the entire lower half of the dashboard is just a big empty heater duct, so much air space and metal to warm up before it gets to the actual distribution. i've thought about direct ducting inside (as in actual plastic pipes) but probably too much effort for our climate (southern uk)