i ran a sports cat on my focus 1.6 se and the management light came on a couple of times but i just disconncted the battery for half hour to reset it, only done it twice in 6 months and didnt do it again...
the piper sports cat had a larger bore found this out when i put the standard cat conv back on to the piper cat back, because i bought the sports cat and cat-back as a complete unit so didnt notice it.
hope this helps, ran a four branch tubular aswell with heat wrap was rather raspy and pops like a goodun
and both cats fitted to standard manifold no problem