Thing is that once you're a jonin, your 'power' doesn't go up that much anymore. You might perfect a technique, or learn a new one, but you're not going to triple or quadruple your power. Unless you pull some shit that goes against the laws of nature.

So, in that regard, [i]sure[/i] a genin could become a chunin, but already be stronger than most jonin.

Thing is that what sets them apart is experience, tactics and understanding. A genin->chunin wouldn't have that where as the jonin would. And so, ultimately the jonin would be better despite lacking the powers.