35i and 35is are interchangeable when it comes to parts, it's just an ECU tune that seperates the two. The catless downpipes are the same on both and if he's suggesting he has to move the fittings for the O2 sensors then he's not fitting the pipes correctly. It's not easy but if you don't getting the fitting angle correct and he resorts to moving the insert then it's likely the rest of the pipes will touch the subframe and rattle.