CB900F2,
I slept on your problem and I want to add the following information:
If Hyundai did their homework when they engineered & sized the heater core, taking into account the following factors:
1. heater blower motor output (cfm) and resultant BTU loss across the core
2. size of the passenger compartment
3. engine head and block coolant passage volume,
then it is possible to maintain control of the engine coolant temperature (in other words, thermostat valve needing to be slightly open), as long as environmental conditions are not too extreme. At some point, if the outside temperature is too low, aka Garak Saskatchewan cold, no matter how sophisticated the design, the heat loss across the heater core (when using the heater, of course) will make it impossible for the coolant temperature to remain elevated, regardless of the thermostat closing.
Having louvers or a grill that blocks more of the outside air flowing over the engine cooling it can definitely help. The more extreme the environment the more modifications are needed. Imagine operating the vehicle in Siberia this January with sub -60°C temperatures (-76°F.)! There any heater use would likely make proper coolant temperature problematic, unless the above airflow blocking strategies were employed.
These small, high compression engines (direct injected 12.0+ CR designs or not) simply do not reject a lot of the combustion heat to the water jacket through the cylinder walls like bigger engines from years prior. That is why many diesel engines utilize electric heaters.
Or, the thermostat, when it should be absolutely closed, is open just the smallest amount. You wouldn't notice its effect when you're not using the heater, particularly in high fan mode, or during warmer weather, but under your conditions and usage pattern, it is just enough to make a difference. Years ago I would remove the suspected bad thermostat from the engine and would, more often than not, see it not fully closed in my hand. Most of them were rated to be .003" open at the nominal opening temperature so it doesn't take much to cause a problem.
Hope this clarifies what goes on in your engine regarding coolant temperature vs. heater output and the balance needed.
And I also hope the dealer says, yep our system is properly engineered to give control down to, say -30°C, your thermostat must be defective, replaces it under warranty, and voila, problem solved!
Good luck,
Gary