My difficult child was much less impulsive on concerta. It made him calmer. I think this is because he truely does have ADHD. For many, the ADHD diagnosis is given when problems first appear. Later, after tests are done, another diagnosis is given.
Concerta, as with all stimulants, can increase impulsivity in any person who does not have ADHD. It is only in people with ADHD that it decreases impulsivity and hyperactiveness. Something in the brain/body chemistry makes this so.
I know in children with bipolar that stimulants, like concerta, can increase cycling.
Each person reacts differently to each medication. The only way to know how a person will react is to try the medication.
I do believe that it is important to rule bipolar OUT before other issues are treated. My reasoning is that so many medications can make a child with bipolar cycle worse. It only makes sense to try the mood stabilizers first, to let the child have the chance to stabilize.
This is my opinion, and my experiences with my difficult child. It is different for each of us and even each of our kids.