Okie dokie, did a small experiment to see what works best and here's what i found. I had 3 Pidgey at varying levels and here is how their evolving progressed.

CP 153 - 294 - 524
CP 155 - 297 - 529
CP 329 - 618 - 1089

So that 170 odd difference early on, makes for roughly 500 by 3rd evolution. So seems spending time to power them up early will pay off in the long run