In theory an O2 sensor's switching will be affected, as in a computer's programming judges the delay of the signal based on how far downstream it is. Now will this actually cause any issues? Likely not, but its dependent on how the computer is set up, and I highly doubt its that precise. So long as it's a heated O2 sensor (most are) I wouldn't worry about it, if its post catalyst in theory it should be the same distance from the catalyst for similar reasons, but again I highly doubt some adjustment will make much difference.