Ditto will copy the EVs as well, excluding HP EVs.
The nature of the Ditto doesn't matter because Imposter copies the stats and stat changes, therefore the nature doesn't matter.
You would indeed speed tie with the Ferrothorn, but most Imposter Ditto's hold a Choice Scarf, outspeeding the opposing Poke, unless they carry one as well.
>Imposter copies the target's appearance, cry, moves (which all have 5 PP), Ability, stats (except HP), stat changes (but not stat multipliers such as with Choice Specs), weight, and Mega Evolution or form (regardless of held item).
>Imposter does not copy its opponent's IVs (for the purpose of Hidden Power) or gender.
Yes, the Ditto would be equal to the Ferro with 3 maxed stats.
Considering all of this, the only stat you can invest in with it affect you at all is HP.
Source: Knowledge and Bulbapedia