do you have a scan tool capable of pulling the engine coolant temp sensor voltage? If so, it's easy to test to see if the thermostat is working correctly. If not, and you have a digital multimeter, you can still test it; however, you'll need to make a jumper wire or backprobe the sensor.
If you want to go that route, I can pull up the voltage parameters from the factory manual. Thermostats are one of those things I wouldn't change unless you know its bad. In my experience you're just as likely to get a bum one out of the box. I will say this car takes a while for the coolant temps to warm up--but if you've noticed a change, you should probably investigate it.