The difference is 15 more attack, 15 more defense, 15 more stamina. This is fixed for each Pokémon.
Speaking of percentages, it depends on the base stats of the Pokémon species.
A Magikarp has 42 base Atk, 84 base Def, 40 base Stamina, so the difference is the 27%.
A Mewtwo has 284 base Atk, 202 base Def, 212 base Stamina, so the difference is only the 6,4%.
Generally, for an evolved pretty strong Pokémon, the difference is around the 10%.
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IV´s Question
Which is the difference betwen a pokemon with perf IV to one with worst IV?
If you can say me a percentage.
Which is the difference betwen a pokemon with perf IV to one with worst IV?
If you can say me a percentage.
Which is the difference betwen a pokemon with perf IV to one with worst IV?
If you can say me a percentage.
Which is the difference betwen a pokemon with perf IV to one with worst IV?
If you can say me a percentage.
Which is the difference betwen a pokemon with perf IV to one with worst IV?
If you can say me a percentage.
Which is the difference betwen a pokemon with perf IV to one with worst IV?
If you can say me a percentage.
Which is the difference betwen a pokemon with perf IV to one with worst IV?
If you can say me a percentage.
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Answers
Currently, because stats are determined by a common "CP multiplier", which at level 40 amounts to a little more than 79%, the difference between a level 40 pokemon with a 0 attack IV and one of the same species and level with a 15 attack IV is actually about 12.
This could change if they raise the pokemon level cap.
