Going from the Exist to an INK is quite the drop in cover strength. Not sure I would go that far down. An Idol Pearl might be the better bet.
As for something that goes a little harder that shouldn't be too difficult. The Exist is a really smooth ball with a gradual move to the pocket. The PhysiX, Halo Vision, UFO, or Omega Crux would do the trick if you're not looking for the long-and-strong pearls. Basically anything high-end in the Storm/Roto lineup besides the SPEC covers.