From what my flatmate told me, he had a Magikarp that was maxed out, then when he Evolved it into Gyarados, it was also maxed out.
I haven’t done this myself yet though, but both our thoughts are that it would be cheaper to max out the pre-evolved state first, then Evolve it.
This is contrary to what I first thought I’d do, but it makes sense. If you look at the bigger picture, we think it’s better to do it that way instead of just wanting to get the evolved state straight away, but a weaker version of it.
I can now confirm this myself. My pre-evolved Magikarp was maxed out at 164, then the resulting Gyarados was also maxed out, at 1741, at an increase of 10.6 times, which is standard for that Evolve (anywhere between 10 and 11 times)
Just to close this issue, now that I know more… Leveling a Pokémon costs the same poor evolution as it does post evolution. Therefore it is far better to level a Pokémon after it’s fully evolved, so you don’t waste candy and stardust leveling a Pokémon that will later develop a rubbish moveset.