So you are sure it's leaking down the barrel?
If so, it is USUALLY to bolt or breech o-rings.
I have found that some of the SP o-rings don't have very good tolerances, try swapping out the o-rings in the breech for fresh replacements.
How big is the leak?
Is the bolt being held all the way back?
I'm assuming this Ion worked fine with the Orange nano extreme kit before it was meched?
I actually have both an Ion with the orange extreme kit, and a mech, but not in the same gun.
From my understanding though, it should work.
Because it leaked with both bolts, I would suspect the o-rings in the breech...
ZDSPB has great Ion info: ZDS Paintball - Ion Troubleshooting - Leaking/Shooting