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The CP value takes the Pokemon's stats into account, so in general a higher-CP Pokemon is more powerful than a lower-CP one, regardless of evolution tier. (This of course isn't strictly true because CP is a flawed metric, but it is at least roughly comparable.) However, keep in mind that evolving a Pokemon will cause its CP to spike because the evolved form will have higher base stats, and the evolved form will usually have stronger moves.

This wouldn't be true in the main-series Pokemon games, where the attacking Pokemon's numeric level is used in the damage formula. A higher-level lower-tier Pokemon will still carry an offensive advantage over a lower-level higher-tier Pokemon; a level 100 Pokemon effectively has twice as much Attack and Special Attack as a level 50 Pokemon with the same stats.

But Pokemon Go doesn't use that damage formula, so it doesn't matter how full the level gauge is -- the Pokemon with the better stats and better moves will do better in battle, regardless of level or evolutionary tier.