I think Siro wins this matchup in 6, and would be more likely to win in 5 than in 7.
GH doesn't have a defensive answer to the awesome guard combinations that Siro can put out. Westbrook and Mitchell would be awesome, but I think you would try and split those two like Houston did with Paul and Harden last year, and pair them with Harris (who is an awesome compliment to both of them). I also think Siro could do some potent small ball lineups with all 3 at once (e.g., Westbrook/Mitchell/Harris/Millsap/Randle or Westbrook/Mitchell/Harris/Roberson/Millsap). Imagine Westbrook pick n roll with Randle rolling, and Milsap, Mitchell, and Harris spaced from 3!
Also, I don't think that GH could put together a good 2 way team as most of the players are weak on at least one side of the floor. On offense, who are you putting around Harden? Gasol might be the best spacer in the starting lineup and he is not the defensive player he once was. Green would also be less of an offensive threat if there aren't great shooters to kick to on the short roll.
Finally, I think that by the end of the series, none of the centers are on the floor in crunch time. When that happens, I think Siro has the best options.