As you have suspected your unlikely to notice much difference if any.There maybe some on here that will talk of noticing a loss of power or talk about cadence, but frankly you here a lot of nonesense on here.

If your short like me – yes
If your tall – no.

Do you think your wife would notice an extra 5mm

Damn right she would.

I moved to 170mm due to sore knees. I did notice the difference on Tarmac in terms of spinning. It also allowed me to line up my knee over the pedal axis more easily. 175mm cranks are pretty standard on most bikes and really don’t take in to account a rider’s size. It’s certainly worth checking out this sort of biomechanics stuff online.

it depends on the length your legs. I’m 179cm I’ve always been on 175mm cranks. I gave it a try last year (had taken the same but shorter cranks from the wife’s bike). When I spun I felt like I’m spending some of my effort for nothing and lap times were worse.

That 5mm could easily come from a slightly different position of your foot in the shoe

Not exactly true. When a foot is in the lowest position its fore/aft position doesn’t matter. In case of shorter cranks one needs up the saddle for the millimetres of cranks length difference too.

I had 170 on the left and 175 on the right for a couple of years after some stripped pedal threads forced a rummage in the spares box. I never noticed any difference.

Running 175mm as opposed to 170mm is basically the difference between running a 35-tooth chainring instead of a 34-tooth, but with less ground clearance on the cranks rather than the chainring.