Bopara no chance, not good enough against the top attacks and England won't go far enough for him to have enough games anyway. Duminy will probably be batting at 5 which is a bit too low to be top run scorer, also if SA get a strong platform there's always the possibility of Albie being promoted ahead of him in a few games.
My money would be on Gibbs, looked like he really had this format figured in the IPL, is in the best mental state he's been in for years (always a big issue with Herschelle) and most importantly he opens, expecting SA to get to the final too so he'll have plenty of games. Suprised no one else had suggested him TBH.
Other good bets are Raina and Gambhir, Raina bats at 3 so he's got a shot, has looked awesome recently. Picking Gambhir on past performances, and again he opens in a strong side so he has max opportunities. Ponting isn't a bad shout, but I just don't think he's good enough at one day cricket any more, 3 years ago he'd have been a prime candidate. Warner is a bit too inconsistent, but one big innings could seriously boost his numbers. Smith's looked out of sorts this year, but is the type of player that could score consistently, has an awesome record in England too.
There are a lot of players who could do really well like Taylor, Gayle, and Symonds, but they either don't bat high enough or simply aren't consistent enough or play in too weak teams to be top runscorer.