I recently purchased a helix downpipe and RRE rally cat. It didnt occur to me until later that it might not fit with my HKS hipower. Because I know the hipower has a reduction just before the cat. Am I wrong and it should fit together?

and also maybe someone knows if the O2 wire is long enough on an IX to reach to the bung on the cat back? The rally cat wont have the bung which was on the stocker.