"Max IVs" does nothing to the actual IVs. However, for the purpose of calculating the final stats, IVs are treated as if they were 15 (the highest in GSC). This means that Hidden Power isn't always Dark, Color variation still works, and so on. Basically, treat it as if all of your Pokémon are Hyper Trained in all stats. Also, keep in mind that the highest stat there assumes a nature that gives a boost in said stat, for example a Modest Mew with 15 DVs (or 0-15 DVs but with "max IVs" on) and 252+ EVs at level 100 would have 328 Special Attack, but a Quirky one would have less.