I will tell you this.
I am level 30 . And i have 2 evolved magikarps witch resulted in this :
(Note that i have used stardust to fully power them up both)
Garydos_89IV = 2334 CP
Garydos_69IV = 2283 CP
So there is a 20IV diffrence which resulted in a 51 CP diffrence.
I think you rather want to find a ballance with the CP/IV.
Its not worth powering up a 100IV magikarp with 10 CP IMO.
As a note: i would use priority CP over IV while under level 40.
Once you get to endgame , you can start and powering up perfect IV's. But untill then , you will just lose your stardust pointlessly.
Imagine at level 40 using maybe 1mil stardust ( just a random number... i have no idea how much it would take ) to power up a 100IV from 10CP to max....
Probably best to keep Highest CP till then. And when u hit 40... you can start replacing them.