As soon as the user comes into battle, it Transforms into its opponent, copying the opponent's stats exactly, with the exception of HP. Imposter copies all stat changes on the target originating from moves and abilities such as Swords Dance and Intimidate, but not from items such as Choice Specs. Imposter will not Transform the user if the opponent is an Illusion or behind a Substitute.
Imposter does not copy its opponent's IVs but it does copy all EVs except for HP. - Bulbapedia
Main point here, Copying the opponents stats exactly, with the exception of HP.
And then it goes on to say that it also copies stat changes, I tested this on PO, by Baton passing Speed boosts to my Ditto ( 4 ) And and on the other side, I had a conkeldurr with a few bulk ups, When Ditto went in, I looked at his stat's and stat changes, and they were to Attack and Defense, but not Speed.
So tested and PO approved, Bulbapedia certified, Ditto won't keep his baton passed stat changes, he will adapt to his opponent's.