I do agree with you that it is a, somewhat, miniscule amount, in the end. I do want to clarify that I am NOT meaning that your PSU is trying to produce 850 watts all of the time and if you're only using say 650W then the excess 200W is wasted.
I AM saying that the efficiency curve does go down the less you use, and there is a sweet spot. So yes, when your efficiency goes down it will affect your heat output and possibly your electric bill (but honestly you probably won't notice).
Take this for example: These numbers I have made up but hear me out.
Say your power supply can achieve 80% efficiency at 850W. At the wall, it will pull about 1062W. The difference is 212W, where it is waste and in the form of heat.
So say this same power supply can achieve 70% efficiency at 650W. AT the wall, it will pull about 928W. The difference is 278W, where it is waste and in the form of heat.
If you attain the BEST efficiency at the most optimal range of your power supply, you will cut down on heat AND waste of electricity. Also keep in mind, these are values at FULL LOAD, whereas most home computers are sitting IDLE. Therefore, your efficiency can be cut even lower. So I again recommend, do not buy a power supply that excessively exceeds your power requirements unless you plan later to add components.