Your getting the car tune so they can shut the rear o2 off so you don't get a code. But depends where you get the car inspected if it passes. Up here I've never had a problem but not sure how things are where you live. If you want just get the catted one. I still think I'd buy the MAC stuff if it was me though.
If you already got the bbk headers get the catted one if you are worried. It's alot more $$ but might be worth it in the long run with emissions down there. I never have ran cats on my Mustangs but I don't have strict testing around here.
when i got my car inspected he just checked if i had a check engine light after that he just pluged in my comp and thats it he said "he wanted to see if i was getting a check light from my muflers"