Does Siakam make that much of a difference?
Indiana is already #1 in offensive rating.
Siakam is an improvement for them defensively. They are 26th in defensive efficiency and poor defensively at the PF spot he'll be taking. That said, Siakam is a good but not great defender. He is similar to Jaylen Brown. Capable of playing strong one-on-one D but often drifts on team D and makes mistakes. He is not a game-changing defender. A good cog in the machine but not more than that.
Nor is Siakam a great rebounder. More of an average rebounder. Usually around 7 rebounds a game. Okay but not great. Especially for someone with his size, length and athleticism. Doesn't get many steals or blocks either.
Scores efficiently in the paint but is otherwise a poor shooter who hurts spacing. Main offensive advantage is his passing and handles. Indiana already have Haliburton for that. I'm not sure how much he'll really add to that their offense. Has a habit of ball-stopping / stagnating the offense as well to setup his drives.