The built in heater in the car actually peaks at 5.5kW during warm up, although it typically settles down to about 3kW once the selected flow temperature is reached. (The temperature knob controls the flow temperature of the water through the heater matrix, not the cabin air temperature - it's up to the driver to manually adjust the heater temperature to regulate the cabin temperature)
Even with this power available it takes about 5 minutes to heat the heater fluid from 0C to 60C (maximum) so it would take an eternity to heat at 1kW, and there's a very strong chance that you simply wouldn't be able to get the heater matrix up to a useful temperature at all with 1kW with cold exterior air blowing through it. If you can't get the flow temperature up to at least 40C you're wasting your time, as this is the lowest setting that will give any useful warmth in winter conditions. (Enough to keep the window clear)
I agree that a 12 volt heater simply isn't up to the job, especially with the added layers of inefficiency of having to heat a loop of water and then heat the mass of the heater matrix itself.
If you really need the extra range and heat in winter a better (although not very PC!) option is a small Diesel/Petrol based heater to heat the heater fluid circuit. A number of people have installed them now and you can get nearly summer range in winter by using one. I was on the verge of installing one myself however the combination of the high up front cost and a recent decision to sell the car this year has nixed that plan.