Just curious, but why would you think a colder plug is required?
I mean, with a 180 t-stat you are already have a lower engine temp, and a colder plug will make things worse.
The basic theory of spark plugs are 2 folded. One is to ignite the A/F mixture and the second is to transfer heat away from the combustion chamber.
The second part is considered the "heat range". It's a fine line on when to go up or down with heat range. Best indicator is the plug tip. If it looks normal, your good to go, ceramic blistering or partial tip melting is too hot, and black or fouling is to cold.
Unless you start throwing HAL codes for a misfire, I wouldn't be to concerned.