i have read this a few times in a few different places and it makes sense. but it is not my experience. all three of my rovers run cooler with the 180 degree thermostat. we do all kinds of driving in them. city with and with out the A/C and highway with and with out the A/C. the coolant temps as monitored by a UG and an app are consistently lower than when i ran the 190 degree thermostat.
I would place more weight on the possibility that the aftermarket body and disk open more fully (i.e. are less restrictive) than the stock unit. At 195 degrees, a 180 degree t-stat is fully open, and a 190 degree t-stat is fully open at 205. Above 205 degrees, both units are fully open and allowing 100% coolant flow. The only restriction would be the mechanical design and flow capacity of the body and disk, and the travel allowed by the heat motor and spring. That scenario is quite possible and logical.
At the end of the day, if you live in a warm climate, you could ostensibly remove the t-stat completely and have unrestricted coolant flow. Even then, if the engine wants to run at 210-215 degrees, that's where it's going to run.
ETA: As an aside, I own a C7 Z06 Corvette, and one of the huge debates that rages on and on in the Vette forums, is the use of 160 degree t-stats. Do you need one? Do you need to alter the tune to use one? What happens in cold weather (cars like to throw codes if the ECU reads too much throttle/boost before the car reaches OEM tune operating temperature)? Which one to use?
Turns out lots of guys were using a Mishimoto aftermarket 160 degree stat and not seeing any difference whatsoever, while guys using other brands WERE seeing significant differences. Turns out, the Mishimoto utilizes a rubber ring gasket that both seals the unit to the housing that wasn't integrated into the t-stat, and was significantly larger. The gasket was actually restricting coolant flow, so even though the unit was opening earlier, it was allowing less coolant flow and negating the benefit:
Mishimoto on left, Racecraft on right.
I understand these are drop-in units, versus the integrated t-stats in a Disco, but the principle of flow restriction would still apply.
Now, if you're TRYING to run the engine at a temperature lower than design temp, that's a different story. Land Rover purposely installed 88C t-stats in these trucks because they wanted them to run in the 200-205 degree range (for a host of reasons). You certainly can get the coolant temp down consistently if you run a colder stat,