ElektroFreak is making a test like this, without TEC but with a very good cooling. I think the lifetime wouldn't change a lot with some degrees colder...
Well, if the worse diode lasts only 1500 hours, that's still very acceptable
But yes, it's impossible to tell how's the diode you're testing when testing only one. But if you take +-20% of the lifetime, you shouldn't be so far from reality!