My understanding (from a couple of people I know who were involved with the movie) was that the first pass (ie at the end of filming in July of 201) of the movie was disappointing, but serviceable (ie make its money back in the theatres and turn a profit on streaming and DVDs). Test audiences' biggest problem was that they didn't know many of the various minor villains (ie Killer Croc, Deadshot and Captain Boomerang) and were confused by Harley Quinn because in the cartoons, she has a definite costume and doesn't use a baseball bat.
Then the trailer was created which made it seem like a *much* different movie than it actually was. The trailer, (see here: https://www.youtube.com/watch?...) generated a lot of buzz due to the energy the characters gave off which wasn't in the original film. The Warner suits realized that there was a disconnect between the trailer and reality, so early last fall, they ordered a script update with reshoots so that the movie would better match the trailer.
The script changes were complete and new filming was finishing up in Vancouver when Deadpool came out and did not affect the final version of the film. It may have confirmed the Warner suits' that they were on the right track, but Deadpool didn't force the decision one way or another. I was told in December that Warner had managed to turn a fair movie that avid comic book readers will like into a "shitty" one that wouldn't work for anybody with the script and filming changes.
The DC Universe movie problems go a lot deeper than trying to copy Deadpool or Avengers movies - it basically comes from a lack of central planning that Marvel/Disney/Sony have in spades as well as difficulty in listening to the casual movie goer.
One of the friends, who's a PA on Suicide Squad, said that fanboys know the characters and that's who DC/Warner tends to listen to rather than somebody off the street who has heard of Batman and Superman but can't name anybody in their rogue's gallery other than The Joker and Lex Luthor and needs an introductory movie for them and the universe, the same way Marvel does.