I can’t really answer your question as I’ve not owned/used all models. However I bought the 8i5 as it seemed to be the sweet spot, price-wise (was about £50 more than the 8i3 but £100 less than the 8i7). It’s working well for me 
I have the Nuc8i7BEH. With an Acourate convolution filter and upsampling to DSD256 my signal path shows a processing speed of 2.0X feeding just that single stream (24 bit file). While I don’t use upsampling very often, that isn’t too crazy of an ask.
I would be a little cheesed if I bought an i5 and couldn’t stretch it to do what I need. You probably know if you’ll ever need something like that, but you might not (a lot of people have never tried convolution filters). So I’m of a camp that you buy up on today’s technology to protect yourself against tomorrow’s needs.
My Nuc is in the basement and I feed ethernet to my DAC, so fan noise isn’t an issue. When my amps (Pass amps) are on, they draw 300W each. So the power draw of an i5 vs an i7 is sort of trivial in the bigger picture.
This does make a bit of the mockery of the “Just get a 7i3, that’s as much as 99% of people will need” line!
So far, the hardest I’ve seen my 8i5 work was whilst sending DSD64 downsampled to 88.2 with volume levelling - for which it was processing at about 6x speed but for the most part it’s way higher than that or usually with no processor speed stated (which I think means it’s north of 100?)
It does. But in most cases I’m just running convolution only and it’s going between 65X and something over 100.
A LOT of people will be perfectly happy with an i3. I just couldn’t go that route because I’d feel boxed in.