Efficiency? Worse. Power? Way more.
Big thing is going from usb 3.0 or really weak microsd to sata ssd or especially m.2 nvme.
Pi4 has a72s which are weak, like clock for clock on par with sandylake or even nehalem, and they aren’t clocked near as high.
Pi5 is closer to broadwell, maybe Sky-lake, which is starting to hit tolerable performance.
Not much, I have services that run both externally and only over wg.
Only issue with wg is sometimes I have to shut it off for things like multicastdns, or otherwise that try to look around the network or wifi.