a J is way too steep. I had one. Going from a I to a Hooker, the Hooker is the way to go, going from a Concorde or Skat Freeride Swirl to a Hooker, Not enough change to justify the difference.
Solas S (14/17) is a better choice for a stock 650. I (14.5/18) is the next pitch up in the series.