[Steve Tinker]

Your water pump / thermostat and coolant have lasted 13 years, so f you are "proactive" and replace all of the above, then logic says that you will be good for the next few years - depending on your annual mileage.
If these engines weren't so allergic to high running temperatures and your coolant efficiency tested out OK, I would say leave as be, but preventative maintenance is high on any professional piece of equipment.
A lot of people mistakenly think that the low temp T'stat makes the engine run too cool for everyday operation in temperate climates like Canada. In fact the 'stat only opens sooner than the original, so giving the engine access to the total cooling volume earlier.
Perhaps we should have call it the early opening thermostat instead of the low temperature thermostat