The World Series will have very little impact. Haunted Mansion Holiday and A Bug's Land are doing much more for the crowds. The locals may be staying home during the games.
We were there Fri-Mon and saw nothing of Twins fans, so I wouldn't expect any out-of-town SF fans either. Outside of the home team's season ticket holders, very few regular folks are able to get their hands on WS tickets so it won't draw many people from out of town. You might see some celebs dropping by though (we saw Randy Johnson on Sunday). Lots of happy people wearing red. Probably as happy as I've seen DL in years...
I wouldn't change my plans...whatever effect it has will only add to your visit.