I had a Qulava, and I thought it was cooler then Typhlosion. At level 40 I finally let Quilave evolve, when this Qualave evolved into Typhplosion, all its stats were recalculated, as if the Pokemon had been a Typhlosion sense level 1. Stalling an evolution has NO NEGATIVE AFFECTS ON STATS, in fact the recalculation is so drastic, that nintendo is afraid the process would glitch the game if allowed to take place mid battle, and is why Pokemon can't evolve mid battle. The game takes the level, the value of the Evs and Ivs, and substitutes the Pokemon for its evolved form, if you hacked the game and told it a mewtwo evolved into a magikharp, you would get a magikharp as weak as any other. the only harm in stalling an evolution is the possibility that the Pokemon's evolved form learns different moves.
I had the same question