Unlike some old second hand, new hardware is more powerful and energy efficient.
So... if I compare an N305 with an 8th gen i3 CPU it is about 2$/year in savings when it comes to power. Since a brand new N305-based machine will sell for at least 150$ more than a second hand HP Mini i3-8300T that means you've to run your N305 for 75 years to actually reach break even.
Look, I like the N305 but I would never get a cheap ass board when I can get a reliable machine with an older CPU like that i3 for a lot less money, it just doesn't make sense. Power consumption is a nice metric to throw around, but once you run the math...
Besides, just google "minis forums uefi bug" and you'll see. Those machines are about luck, you may have good results a few times but you'll eventually get burned by some board with software or design issues.
CW p-5 (N305) as my router (opnsense)
Frankly, do you really need opnsense? If you were to remove that and just grab any decent router, even old hardware, like the R7800, and load it with OpenWrt you would be spending a lot less on power. Go ahead, name a opnsense feature that OpenWrt doesn't have. :)