Take two otherwise identical B-bodies. Equip one with a 160°F thermostat, the other with a 180°F.
Drive them under the exact same conditions (pretend this is possible). Say, your freeway scenario, which I will assume stayed above 40-something MpH, as the fans never came on. That's because the PCM was programmed to leave the fans off above 40-something MpH because at or above 40-something, nature exceeds the fans' performance, making them redundant.
The one with the 160°F thermostat begins to open @ 160°F, and FULLY opens by 175°F.
The one with the 180°F thermostat begins to open @ 180°F, and FULLY opens by 195°F.
To make it short, the car with the 180°F stat will blow air through the heatercore @ 20°F warmer than the car with the 160°F.
3) For the winter, you can take off the airdam under the radiator.
4) Hook up your AutoXray, disconnect the fans, and watch the Coolant Temp Sensor's readings rise while operating the passenger heater fans.
(Reconnect both fans before your CTS reads 230°F.)
If, compared to your present heater's performance, you don't notice a significant improvement in your comfort levels before the CTS reports 215°F, flush, backflush, and reflush each major component of your cooling system separately (this would be the ideal, so that you don't force contamination from one part into or through the other).
If it does improve, then your heatercore's performance is just being held back by cold coolant.
Several years before the GDL, a date told me to turn back so we could use her car, as her heater 'wasn't broken like mine'.
Since then, I use a 180°F thermostat with appropriate fan settings for the winter.