When Granlund is lower down on the wall he is very good at keeping possession( I.e. Ability to stickhandle in a phone booth), but not much of a threat to penetrate into the scoring areas except by passing, so defenders are content to let him kill time on the perimeter. Higher up the wall in the umbrella position he is less effective, as his lack of a shot from distance makes him even less of a threat to PK defenders. That is where Dumba or Spurgeon would pay dividends. Both have very good to great shots at distance, and both are good at weak side pinching, making for an offensive threat when Parise or Vanek have the puck.
Both are decent(Dumba) to very good(Spurgeon) passing the puck from the umbrella. I do prefer to have Spurgeon play Suter's spot on the #2 PP by a long shot over Dumba, but it's not a biggie, as the #2 PP gets little TOI.
The key thing here is not to have two players in Suter and Granlund who over handle ( slow down)the puck movement. You have one puck handler inSuter, a gunner on either side in Parise and Dumba, another puck handler in Koivu, and a net front man in Vanek who can be a shooter OR a puck handler should they decide to switch spots and confound the enemy. Thing is, Vanek barely gets to touch the puck because not enough shots are getting taken. It's analogous to a C in basketball not getting passes down low in the key, so after a while you stop fighting for position. Seems like Granlund and Suter are always waiting for the perfect shot... Most goals aren't scored on pretty plays, but deflections and rebounds. If the shot isn't hard enough or the release is too slow, it lessens the chance of the puck getting through, and allows defenders time to get into position.
Btw, Granlund can always play the Koivu position in the slot, though I think MK is a better option with his FO skills and bigger body.