I see it like this, *IF* they are deadset on making a JL film, then they should use the same superman thats in MOS.
The Flash should be faster, no contest.
W.W. should be almost as strong, but because she has magical aspects she could stand toe to toe with supes,
GL should be almost as powerful, maybe just as powerful.
M.M. just as strong and also invulnerable,
and Batman should be like he was in the JL animated series, ie, able to go toe to toe with even the most powerful villains bcause of his martial arts training and agility, and also be able to outwit them with his mind.
Superman would be the most powerfullest tho once you added up all his powers like heatvision, X-ray vision, superhearing, speed, strength, etc etc.
They will either have to weaken superman down to their level, or bring them up to his. I really don't want to see Supes weakened for the sake of the other characters, so I prefer they be brought up to his level.
Bring in a Darkseid or Doomsday to justify such powerful heroes. It's not rocket science.
In fact, I'd like to see a villain so powerful that it would take 3 or more supermen to defeat him, hence the need for a JL.