Hi mate, if you have the time and money, anything can fit anything, thats what I say. I have no idea if the 500 abarth box will fit. Not had a look at both bocxes together, someone else will be able to shed light on that.
Az for sports cat, there is a sports cat available but the only direct replacement is a supersprint one thats one the other side of a grand. If you wanted, you could get the abarth turbo and then you can get a direct replacement from TMC for only 310. If TMC can confirm that the cat from the abarh is the same as the sporting, that wpuld help me.
Exhaust boxes are few and far between. Theres not alot of choice unless you get a universal box.
As for titanium (i seen in your post in the other thread) Titanium is jsed to reduce weight of the exhaust system by around ~10kg, not a huge saving but anything is a bonus that reduces weight.
Any more questions mate?