You should also look at Optimizing Blue Iris's CPU Usage to get the most out of the hardware you have.
But if you are trying to sort the minimum CPU and GPU you can get away with or trying to justify the cost of an upgrade I should note it is almost impossible to get a definitive improvement number of X setup over Y unless you have both running side my side looking at the same cams. Things like the amount of motion the cams are seeing, overlays and other features in use factor in too. However this page Blue Iris Performance Statistics has a sortable and filterable table of config and load data collected from around 1400 user servers. But that still just gets you in the ballpark.
For instance I have this i7-4790 I bought used almost 5 years ago now with 10 outdoor cams that was running 50-60% CPU when I looked at it the other day when it was relatively calm. Today it looks like this, maxing out about 72%
|Iris2 load and config|
I built a i7-8700 with those same 10 cams to replace the 4790 PC. Even after adding another 6MP cam it was running 20-30% CPU during the same period as the above. Today it is maxing about 21%
|Iris6 load and config|
This one is close to max with 9 outdoor and 3 indoor cams on it.
|Iris3 load and config|
|Iris5 load and config|
And just to show the diff motion can make, this one with 23 active and 1 offline indoor cams that could handle a few more.
|Iris4 load and config.|