I am going to strongly agree with RedCelicaTRD on this one.
I know for a fact from previous datalogging experience, that the T-VIS will open with EVERY touch of the throttle until the coolant temp reaches 200 degrees Fahrenheit (at the temp sensor in the elbow). That would lead me to believe that anything below 200F, the ECU will see as 'cold start/warm-up enrichment' mode. Also, since the T-VIS will open with every throttle touch instead of at the normal 4100RPM range, the car might possibly be making less power.
I have a stock thermostat in my 165. I don't know what temp it's rated at, but my car hovers around 200F-210F under normal driving. I've only seen it dip to 199F once (it was 20 below zero that day). That also leads me to believe that the car is not meant to run colder.
Like other people have said, if there is overheating issues (and the thermostat is 100% in good working condition), than installing a low-temp thermostat is just a bad excuse for a cooling solution. A properly functioning cooling system can run hard all day long and not have an overheating issue.
All that being said, we all know that the car runs perfectly fine with the T-VIS opened all the time. A lot of people even remove it. So if that is the only consequence of running a cooler thermostat, then by all means go for it. I also acknowledge that TRD must have created these low temp thermostats for a reason, and not just for a moneymaking gimmick (we all should hope!).
I also have to acknowledge what etantshi said, that even with the colder Thermostat, the coolant could possibly be at normal temp, once it reaches the sensor. I can't verify whether or not this is true because I don't run a low temp stat. So I do understand what you are saying etantshi, I just have no way to know if it's true or not.
As a final note I will say this: (and I am not talking to any single person here, just the general public) If you are having overheating problems, don't buy a low-temp thermostat, because the bottom line is you have another problem!. Buy a stock one, and if you are still having a problem, check your fans (stock are better than those thin-line things), check your coolant level, check you temp sensor, look for signs of a blown HG, look for leaks, check your radiator for cracks or blockage. When you search out and find the real culprit, and you have that "AHA!" moment, you will thank those of us that give you a hard time. Good luck!