To put this into perspective for you, if your NAS sits at idle for 90% of the time (probably true) and an older CPU is 50w (kinda high, but maybe) and a newer CPU is 15w, over an entire year it will save you around 305.76 kWh. Average price per kWh in the USA is 12.89¢. So over a year a new CPU can reasonably save you around $39.41. So it’s not nothing, but it’s nothing crazy, but lower idle wattage = lower temp = components last longer, which is the real savings.

If an older CPU is only gonna last you 5 years, when a new might last 10, you’re going to save almost $400 in energy and generally a CPU today is going to be cheaper than a CPU in 10 years (probably^tm).